fixed bug in defining embed dim for UNet1D

[SammyAgrawal]

What does this PR do?

Fixes #12110

Hi! First time contributor to diffusers, apologies for anything I'm doing wrong in the PR. I noticed what I believe is a very simple and small bug in creating 1D UNets. A value is hardcoded that should not be. I noticed this while trying to debug why my UNet kept giving me shape errors. I just transitioned from using a 2d model to 1d for my personal use case, and this was causing me a headache! I am basically solving my own "Good First Issue" (linked above) noticed during my usage. I believe the Issue itself explains the problem and solution pretty straightforwardly.

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Tagging @sayakpaul @yiyixuxu @DN6

[SammyAgrawal]

I should note that although I tried to follow the contributing doc, there were parts that were hard for me to replicate / understand.
For example, should I not be trying to merge into main? I only saw that my fork branch should not be main.

Will happily go back and fix this before review if asked. For example, I ran python tests/models/unets/test_models_unet_1d.py but it failed with:

Traceback (most recent call last):
  File "/home/jovyan/Samarth/diffusers/tests/models/unets/test_models_unet_1d.py", line 29, in <module>
    from ..test_modeling_common import ModelTesterMixin, UNetTesterMixin
ImportError: attempted relative import with no known parent package

Which is unrelated to my modification. I think this means I set it up wrong?