Implementation of left padding for issue #1077

Implementation of left padding for issue #1077. This is based on a suggestion
by @gabrielspmoreira. I am not exactly sure if this change will completely
work, and this is untested due to current failing tests on main on this part of
the codebase. But the motivation of this commit is to start a commit for
comments, suggestions, and revisions on this issue's implementation.

nvtabular/loader/torch.py

@@ -174,8 +174,12 @@ def _get_sparse_tensor(self, values, indices, num_rows, seq_limit):
             sparse_tensor = sparse_tensor.to_dense()
         return sparse_tensor
 
-    def _build_sparse_tensor(self, values, offsets, diff_offsets, num_rows, seq_limit):
+    def _build_sparse_tensor(self, values, offsets, diff_offsets, num_rows, seq_limit, padding=None):
         indices = self._get_indices(offsets, diff_offsets)
+        if padding == "left":
+            indices[:,1] = seq_limit - 1 - indices[:,1]
+        if padding == "right":
+            raise NotImplementedError
         return self._get_sparse_tensor(values, indices, num_rows, seq_limit)