Fix small inconsistency in output dimension of "_get_t5_prompt_embeds" function in sd3 pipeline

[alirezafarashah]

What does this PR do?

This PR fixes a small inconsistency in the output dimension of the _get_t5_prompt_embeds function in the Stable Diffusion 3 pipeline.

Previously, when self.text_encoder_3 was None, the function returned a tensor (torch.zeros) with a sequence length of self.tokenizer_max_length (77), which corresponds to the CLIP encoder. However, the T5 text encoder used in SD3 has a different maximum sequence length (256).

As a result, when text_encoder_3 was available, the prompt embeddings had a sequence length of 333 (256 from T5 + 77 from CLIP), but when it was not available, the returned tensor had only 154 (77 + 77), leading to an inconsistency in output dimensions in encode_prompt.

Motivation and Context

This change ensures consistent tensor shapes across different encoder availability conditions in the SD3 pipeline.
It prevents dimension mismatches and potential runtime errors when text_encoder_3 is None.

Previously, the zeros tensor used self.tokenizer_max_length, which corresponds to CLIP, instead of T5’s longer sequence length.
This mismatch led to inconsistent embedding dimensions when combining outputs from CLIP and T5 in encode_prompt.

Changes Made

• Replaced self.tokenizer_max_length with max_sequence_length when returning the zero tensor in _get_t5_prompt_embeds, ensuring consistent output dimensions whether text_encoder_3 is None or available.
  The same max_sequence_length parameter is already used in the tokenization step of the same function:

  text_inputs = self.tokenizer_3(
      prompt,
      padding="max_length",
      max_length=max_sequence_length,
      truncation=True,
      add_special_tokens=True,
      return_tensors="pt",
  )

• No changes to functionality, inputs, or outputs beyond dimension consistency.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• Did you write any new necessary tests?

Who can review?

• @asomoza (pipelines and callbacks)
• @yiyixuxu (pipelines and callbacks)
• @sayakpaul (general functionalities)

[yiyixuxu]

thanks!

[yiyixuxu]

@bot /style

[github-actions]

Style fix runs successfully without any file modified.

[yiyixuxu]

hey @alirezafarashah
can you run `make fix-copies"?

[HuggingFaceDocBuilderDev]

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[alirezafarashah]

hey @alirezafarashah can you run `make fix-copies"?

Hey @yiyixuxu
I did and pushed it.