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* Updated docstrings of bert_padding.py

Added docstrings for missing arguments in the unpad and pad methods.

* Update bert_padding.py

Fixed spelling mistakes

* add unpad_input_for_concatenated_sequences

* modify docstring

According to the `setup.py` file, only dependencies are torch and einops. But the `bert_padding.py` file requires `numpy` only to multiply the elements of a `torch.Size` object. This change aims at allowing the use of FlashAttention without numpy.