[BUG] IndexError with tensor_sum and rebasin

[n0kovo]

When running with

merge_mode: tensor_sum
rebasin: True

I get a IndexError: tuple index out of range at "Rebasin iteration 1".
It doesn't happen when rebasin is set to False or when using weigthed_sum instead of tensor_sum.

Likely related to #76.

Output:

2024-03-11 10:59:37 INFO [root] Loading: ~/Downloads/stable-diffusion-webui/models/Stable-diffusion/model1.safetensors
2024-03-11 10:59:38 INFO [root] Loading: ~/Downloads/stable-diffusion-webui/models/Stable-diffusion/model2.safetensors
2024-03-11 10:59:38 INFO [root] start merging with tensor_sum method
2024-03-11 10:59:47 INFO [root] Init rebasin iterations
2024-03-11 10:59:47 INFO [root] Rebasin iteration 0
stage 1: 100%|████████████████████████████████████████████████████████████████████████████████████████████| 1131/1131 [00:02<00:00, 504.26it/s]
stage 2: 100%|█████████████████████████████████████████████████████████████████████████████████████████| 1133/1133 [00:00<00:00, 177338.75it/s]
2024-03-11 11:00:00 INFO [root] Rebasin iteration 1
stage 1: 100%|██████████████████████████████████████████████████████████████████████████████████████████▊| 1131/1133 [00:00<00:00, 1748.68it/s]
*** API error: POST: http://127.0.0.1:7860/bbwm/merge-models {'error': 'IndexError', 'detail': '', 'body': '', 'errors': 'tuple index out of range'}
    Traceback (most recent call last):
      File "~/.asdf/installs/python/3.11.5/lib/python3.11/site-packages/anyio/streams/memory.py", line 98, in receive
        return self.receive_nowait()
               ^^^^^^^^^^^^^^^^^^^^^
      File "~/.asdf/installs/python/3.11.5/lib/python3.11/site-packages/anyio/streams/memory.py", line 93, in receive_nowait
        raise WouldBlock
    anyio.WouldBlock

    During handling of the above exception, another exception occurred:

    Traceback (most recent call last):
      File "~/.asdf/installs/python/3.11.5/lib/python3.11/site-packages/starlette/middleware/base.py", line 78, in call_next
        message = await recv_stream.receive()
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^
      File "~/.asdf/installs/python/3.11.5/lib/python3.11/site-packages/anyio/streams/memory.py", line 118, in receive
        raise EndOfStream
    anyio.EndOfStream

    During handling of the above exception, another exception occurred:

    Traceback (most recent call last):
      File "~/Downloads/stable-diffusion-webui/modules/api/api.py", line 186, in exception_handling
        return await call_next(request)
               ^^^^^^^^^^^^^^^^^^^^^^^^
      File "~/.asdf/installs/python/3.11.5/lib/python3.11/site-packages/starlette/middleware/base.py", line 84, in call_next
        raise app_exc
      File "~/.asdf/installs/python/3.11.5/lib/python3.11/site-packages/starlette/middleware/base.py", line 70, in coro
        await self.app(scope, receive_or_disconnect, send_no_error)
      File "~/.asdf/installs/python/3.11.5/lib/python3.11/site-packages/starlette/middleware/base.py", line 108, in __call__
        response = await self.dispatch_func(request, call_next)
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
      File "~/Downloads/stable-diffusion-webui/modules/api/api.py", line 150, in log_and_time
        res: Response = await call_next(req)
                        ^^^^^^^^^^^^^^^^^^^^
      File "~/.asdf/installs/python/3.11.5/lib/python3.11/site-packages/starlette/middleware/base.py", line 84, in call_next
        raise app_exc
      File "~/.asdf/installs/python/3.11.5/lib/python3.11/site-packages/starlette/middleware/base.py", line 70, in coro
        await self.app(scope, receive_or_disconnect, send_no_error)
      File "~/.asdf/installs/python/3.11.5/lib/python3.11/site-packages/starlette/middleware/cors.py", line 84, in __call__
        await self.app(scope, receive, send)
      File "~/.asdf/installs/python/3.11.5/lib/python3.11/site-packages/starlette/middleware/gzip.py", line 24, in __call__
        await responder(scope, receive, send)
      File "~/.asdf/installs/python/3.11.5/lib/python3.11/site-packages/starlette/middleware/gzip.py", line 44, in __call__
        await self.app(scope, receive, self.send_with_gzip)
      File "~/.asdf/installs/python/3.11.5/lib/python3.11/site-packages/starlette/middleware/exceptions.py", line 79, in __call__
        raise exc
      File "~/.asdf/installs/python/3.11.5/lib/python3.11/site-packages/starlette/middleware/exceptions.py", line 68, in __call__
        await self.app(scope, receive, sender)
      File "~/.asdf/installs/python/3.11.5/lib/python3.11/site-packages/fastapi/middleware/asyncexitstack.py", line 21, in __call__
        raise e
      File "~/.asdf/installs/python/3.11.5/lib/python3.11/site-packages/fastapi/middleware/asyncexitstack.py", line 18, in __call__
        await self.app(scope, receive, send)
      File "~/.asdf/installs/python/3.11.5/lib/python3.11/site-packages/starlette/routing.py", line 718, in __call__
        await route.handle(scope, receive, send)
      File "~/.asdf/installs/python/3.11.5/lib/python3.11/site-packages/starlette/routing.py", line 276, in handle
        await self.app(scope, receive, send)
      File "~/.asdf/installs/python/3.11.5/lib/python3.11/site-packages/starlette/routing.py", line 66, in app
        response = await func(request)
                   ^^^^^^^^^^^^^^^^^^^
      File "~/.asdf/installs/python/3.11.5/lib/python3.11/site-packages/fastapi/routing.py", line 237, in app
        raw_response = await run_endpoint_function(
                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
      File "~/.asdf/installs/python/3.11.5/lib/python3.11/site-packages/fastapi/routing.py", line 163, in run_endpoint_function
        return await dependant.call(**values)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
      File "~/Downloads/stable-diffusion-webui/extensions/sd-webui-bayesian-merger/scripts/api.py", line 78, in merge_models_api
        merged = merge_models(
                 ^^^^^^^^^^^^^
      File "~/.asdf/installs/python/3.11.5/lib/python3.11/site-packages/sd_meh/merge.py", line 149, in merge_models
        merged = rebasin_merge(
                 ^^^^^^^^^^^^^^
      File "~/.asdf/installs/python/3.11.5/lib/python3.11/site-packages/sd_meh/merge.py", line 292, in rebasin_merge
        thetas["model_a"] = simple_merge(
                            ^^^^^^^^^^^^^
      File "~/.asdf/installs/python/3.11.5/lib/python3.11/site-packages/sd_meh/merge.py", line 245, in simple_merge
        res.result()
      File "~/.asdf/installs/python/3.11.5/lib/python3.11/concurrent/futures/_base.py", line 449, in result
        return self.__get_result()
               ^^^^^^^^^^^^^^^^^^^
      File "~/.asdf/installs/python/3.11.5/lib/python3.11/concurrent/futures/_base.py", line 401, in __get_result
        raise self._exception
      File "~/.asdf/installs/python/3.11.5/lib/python3.11/concurrent/futures/thread.py", line 58, in run
        result = self.fn(*self.args, **self.kwargs)
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
      File "~/.asdf/installs/python/3.11.5/lib/python3.11/site-packages/sd_meh/merge.py", line 353, in simple_merge_key
        with merge_key_context(key, thetas, *args, **kwargs) as result:
      File "~/.asdf/installs/python/3.11.5/lib/python3.11/contextlib.py", line 137, in __enter__
        return next(self.gen)
               ^^^^^^^^^^^^^^
      File "~/.asdf/installs/python/3.11.5/lib/python3.11/site-packages/sd_meh/merge.py", line 450, in merge_key_context
        result = merge_key(*args, **kwargs)
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^
      File "~/.asdf/installs/python/3.11.5/lib/python3.11/site-packages/sd_meh/merge.py", line 422, in merge_key
        merged_key = merge_method(**merge_args).to(device)
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^
      File "~/.asdf/installs/python/3.11.5/lib/python3.11/site-packages/sd_meh/merge_methods.py", line 43, in tensor_sum
        talphas = int(a.shape[0] * beta)
                      ~~~~~~~^^^
    IndexError: tuple index out of range