fix(granitemoe*): Only create block_sparse_moe if num_local_experts > 0

[gabe-l-hart]

Branch: GraniteMoeAsDenseFix

What does this PR do?

With the introduction of modular_granitemoe.py in #40132, the conditional that allowed GraniteMoe to also encapsulate dense models as a degenerate case was accidentally removed. This is never actually needed for the GraniteMoe architecture directly, but GraniteMoe is reused in GraniteMoeShared and then GraniteMoeHybrid which do need this ability to also encapsulate dense FFN blocks in place of the MoE block.

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Who can review?

@ArthurZucker I believe this came in with your PR for MoE in vLLM, so I'd love your sanity check on this fix.

[gabe-l-hart]

Looks like I need to regenerate the modeling_ code everywhere

[gabe-l-hart]

Interestingly, when I run make fix-copies, the python utils/check_modular_conversion.py --fix_and_overwrite script makes far more changes than I would expect from adding this conditional. In particular, for each architecture in the GraniteMoe* chain, it adds GraniteMoe<qualifier>SparseMoeBlock, then in the __init__ for GraniteMoe<qualifier>DecoderLayer, it adds self.block_sparse_moe = GraniteMoe<qualifier>SparseMoeBlock(config) (without any conditional) AND the guarded conditional block self.block_sparse_moe = GraniteMoeHybridMoE(config).

Looking a little deeper, this seems to be caused by the inheritance from modular_mixtral.py. I've added a clause that explicitly uses delattr to remove self.block_sparse_moe when not used, but that seems a bit backwards. Alternatively, we could not inherit from MixtralDecoderLayer or we could move the conditional up to MixtralDecoderLayer.__init__.

The part that I'm still confused about is why regenerating the modeling_* files is adding these SparseMoeBlock implementations at all. The inheritance from Mixtral was already there prior to my change, so I would expect that those should have already been added unless somehow making creation of self.block_sparse_moe conditional triggered logic in the generation to require that those be added?

[gabe-l-hart]

It appears that putting creation of self.block_sparse_moe behind a conditional does, in fact, trigger the inclusion of those SparseMoeBlock pieces in the generation. I've used an inline conditional now which seems to prevent this.

[gabe-l-hart]

🤦 Ok, all that was because of a bad copy-paste somewhere that had me creating a completely incorrect block for self.block_sparse_moe. Fixed and cleaned up history.

[gabe-l-hart]

One more redo: Based on advice from @ArthurZucker, since there are no models using either GraniteMoe or GraniteMoeShared with the degenerate dense configuration, it's preferable to only have this conditional override in GraniteMoeHybrid where it is needed for various flavors of granite-4.0-* models.

[gabe-l-hart]

It looks like these just got moved during the regeneration. I'm not sure if should be included (to enforce consistency with the generation script) or excluded (to minimize the size of the change).

[Rocketknight1]

Hi @gabe-l-hart, thanks for the PR! You can get the code style tests to pass with pip install -e .[quality] followed by make fixup or make style.

Overall it looks good to me, and it does seem like the zero-experts case was accidentally deleted. Will wait for @ArthurZucker to confirm before merging!

[gabe-l-hart]

@Rocketknight1 Thanks! I'll get it cleaned up and hopefully green today

[github-actions]

[For maintainers] Suggested jobs to run (before merge)

run-slow: granitemoehybrid