Transformer block that supports jagged tensor as input

[yuhuishi-convect]

The standard transformer block supports input tensors that will have same sequence length.
This requires padding the example sequences to the same length in a batch if the lengths are not equal.

The introduction of JaggedTensor does support the variable length examples. Can we support a transformer block that can efficiently work with JaggedTensor or KeyedJaggedTensor as input, instead of converting them to the dense and padded form?

[henrylhtsang]

Hi, this seems like a question for core library https://github.com/pytorch/pytorch