Support regular expression in the mapping arg of copy_model_state

[KumoLiu]

Part of #6552.

Description

After PR #6835, we have added copy_model_args in the load API which can help us update the state_dict flexibly.
https://github.com/KumoLiu/MONAI/blob/93a149a611b66153cf804b31a7b36a939e2e593a/monai/bundle/scripts.py#L397

Given this issue, we need to be able to filter the model's weights flexibly.
In copy_model_state, we already have a "mapping" arg, the filter will be more flexible if we can support regular expression in the mapping. This PR mainly added the support for regular expression for "mapping" arg.

In the example in this issue, after this PR, we can do something like:

exclude_vars = "encoder.mask_token|encoder.norm.weight|encoder.norm.bias|out.conv.conv.weight|out.conv.conv.bias"
mapping={"encoder.layers(.*).0.0.": "swinViT.layers(.*).0."}
dst_dict, updated_keys, unchanged_keys = copy_model_state(
       model, ssl_weights, exclude_vars=exclude_vars, mapping=mapping
)

Additionally, based on the comments of Eric here, I totally agree, we could add a handler to make the pipeline easier to implement, but perhaps this task is no need to set as a "BundleTodo" for MONAIv1.3 but as an enhancement for MONAI near future.
What do you think? @ericspod @wyli @Nic-Ma

Types of changes

• Non-breaking change (fix or new feature that would not break existing functionality).
• Breaking change (fix or new feature that would cause existing functionality to change).
• New tests added to cover the changes.
• Integration tests passed locally by running ./runtests.sh -f -u --net --coverage.
• Quick tests passed locally by running ./runtests.sh --quick --unittests --disttests.
• In-line docstrings updated.
• Documentation updated, tested make html command in the docs/ folder.

[KumoLiu]

According to Wenqi's comments and #6552 (comment), we can leave mapping doesn't support regular expression here, but add a filter_func in copy_model_state which defaults to None.
For filter_func it doesn't matter how it is implemented inside, the important thing is just to return a pair of keys and values that will need to be filtered, and return None otherwise.

Since copy_model_state is also used in CheckpointLoader, we can also easily pass our filter_func into it to help filter weights, researchers can also provide some common-used filters for users to use.

MONAI/monai/handlers/checkpoint_loader.py

Line 138 in ef7debe

checkpoint[k] = copy_model_state(obj, checkpoint, inplace=False)[0]

def filter_swinunetr(k, v):
    if k in [
        "out.conv.conv.weight",
        "out.conv.conv.bias",
    ]:
        return None

    if k[:8] == "encoder.":
        if k[8:19] == "patch_embed":
            new_key = "swinViT." + k[8:]
        else:
            new_key = "swinViT." + k[8:18] + k[20:]

        return new_key, v
    else:
        return None

dst_dict, updated_keys, unchanged_keys = copy_model_state(
       model, ssl_weights, filter_func=filter_swinunetr
)

If you think this works, I can continue with this PR. Thanks.

[wyli]

looks good to me, the test case show the usage,cc @ericspod @Nic-Ma @vikashg please share any further comments

[wyli]

/build