Fix SFT Pipeline when TP>1

[asolergi-nv]

What does this PR do ?

Requires merging #3254 first. This PR includes the last fixes that enable SFT with our current "fake packed" format when using TP.

• Guard the import of SFT tokenizer from megatron.training in build_tokenizer.py from megatron.core
• Remove duplicate code introduced in Hybrid Context Parallel Feature #2282 & Add sequence packing support for hybrid model #2913
• The development of Hybrid Context Parallel Feature #2282 started way before Add sequence packing support for hybrid model #2913. By that time they only considered that max_seqlen was going to be consumed by hybrid CP, but it's also required for SFT. Hybrid Context Parallel Feature #2282 was merged after Add sequence packing support for hybrid model #2913, and in that PR they were setting max_seqlen to None except when using Hybrid CP, leaving the SFT pipeline without that tensor and raising a issue.

I tried this PR merging the changes from #3254 and it's working as expected. @arendu is already working with this fix too.

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[asolergi-nv]

/ok to test 03af6e3

[asolergi-nv]

/ok to test d853d45

[asolergi-nv]

/ok to test 7180ccb

[duncanriach]

Looks good. Thanks