Add conditions_embeddings argument to TransformerBlock, TransformerLayer for DiT (diffusion transformer)

[huvunvidia]

What does this PR do ?

The PR is for introducing an extra argument conditions_embeddings for TransformerLayer, TransformerBlock.
This argument is for providing timesteps embeddings for all diffusion models.
Currently, to inject timesteps embeddings, we need workaround method:

• Wan/Cosmos text-to-video: using attention_mask as a place holder (link), which is not a good practice
• FLUX: overriding TransformerLayer to support that argument, and avoid TransformerBlock all together (link)

Since this timesteps/conditions embeddings are used by almost all diffusion models, it would be reasonable to add official argument for it.

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[huvunvidia]

/ok to test 03d7ba3