Support left padding to forward batch prompts in a single step

[guillaumekln]

Batches of variable-length prompts are currently not forwarded in a single step into the decoder. Only the tokens up to the minimum length in the batch are forwarded at once, and the remaining tokens are force-decoded in the decoding loop.

It could be more efficient to forward the batch in a single step but this requires supporting padding positions on the left. It means each entry in the batch has an offset in position-aware modules:

• softmax
• position encodings
• rotary embeddings
• padder
• MHA values "mask"