Fix redundant prev_output_channel assignment in UNet2DModel

[ahmedbelgacem]

What does this PR do?

Problem:
In UNet2DModel, the variable prev_output_channel is assigned twice in a redundant manner.
Fixes:
Removing the second assingnement

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Who can review?

@sayakpaul @yiyixuxu

[yiyixuxu]

thanks!

[HuggingFaceDocBuilderDev]

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