Add padding to encoder_seq_lens

[kdamaszk]

Without this change we can observe below error:

[rank0]:   File "/software/users/kdamaszke/repos/vllm-fork/vllm/model_executor/models/mllama.py", line 959, in forward
[rank0]:     full_text_row_masked_out_mask = full_text_row_masked_out_mask.view(
[rank0]: RuntimeError: shape '[4, -1, 1]' is invalid for input of size 3

It occurs when one of the requests is removed from the batch earlier. In that case, language model is still working on the shapes padded to the bucketed batch size, while encoder input doesn't. This change is aligning the batch size on encoder_seq_lens to the expected one.

[yisonzhu]

LGTM.