fix: fix mcore sequence packing

[yuki-97]

As title.

Fix nightly test sft-llama3.1-8b-1n8g-megatron-seqpack.

if mbs is None:
    mbs = self.cfg["train_micro_batch_size"]

# before fix
# (
#     data_iterator,
#     num_microbatches,
#     micro_batch_size,
#     seq_length,
#     padded_seq_length,
# ) = get_microbatch_iterator(...)

# after fix
(
    data_iterator,
    num_microbatches,
    mbs,
    seq_length,
    padded_seq_length,
) = get_microbatch_iterator(...)

losses_reduced = forward_backward_func(
    ...
    micro_batch_size=mbs,
    ...
)

[yfw]

Thanks for fixing!

[yuki-97]

Closed since #1654 will fix this as well.