Add sequence packing support for hybrid model

[duncanriach]

What does this PR do ?

Adds sequence packing support for the hybrid model and also enhances --sft, SFTDataset, and pretrain_mamba.py to feed packed sequence data into the model.

Causes a regression in the functionality of --sft on the GPT model; this functionality will no longer be supported. To maintain this functionality, packed-sequence related changes would need to be made to pretrain_gpt.py following pretrain_mamba.py and, ideally, GPT model SFT packed-sequence convergence testing would need to be run. I believe this functionality will be getting added by PR 2282 from @parthmannan, which will be merged after this current PR.

This PR also does not contain functional tests for packed sequence support in the hybrid model. I believe that there are currently also no functional tests for packed sequence support in the GPT model.

See internal MR 4403

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[Phlip79]

/ok to test a2f0e20