fix generation for statically reshaped diffusion pipeline

[eaidova]

What does this PR do?

in case if pipeline was reshaped to generate images of given size and this size is different from user requested or used in pipeline by default generation raised shape mismatch error

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?

[HuggingFaceDocBuilderDev]

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