Fix TransformerConfig validation for mixed dense/MoE upcycling

[rkteddy]

What does this PR do ?

Fix TransformerConfig validation for mixed dense/MoE upcycling (--moe-layer-freq + --moe-use-upcycling):

In a mixed model, dense layer configs inherit the global moe_ffn_hidden_size while having num_moe_experts=None. The existing assert in TransformerConfig.__post_init__ raises an error in this case. Replace the assert with a warning and set moe_ffn_hidden_size = None for dense layers.

Related: #3646 fixes the upcycling state dict conversion for the same scenario.

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[asolergi-nv]

Can we make this check more robust at least by checking those flags + explicitly warning in which scenario we expect to see the warning? Thanks!

[rkteddy]

Thanks for the suggestion! Updated the check to also inspect moe_layer_freq:

• If moe_layer_freq indicates a mixed dense/MoE model, warn and reset moe_ffn_hidden_size, this is the expected scenario where dense layers inherit the global MoE config.
• Otherwise, raise a ValueError when having moe_ffn_hidden_size set without num_moe_experts is a misconfiguration.