allow tensors in several schedulers step() call

[catwell]

What does this PR do?

Fix typing of the timesteps argument of schedulers taking an integer to also allow tensors. The timestep is passed to _init_step_index which deals with this case.

I did not change the docstrings to "int or torch.Tensor", I can do it if you want (but most other schedulers that already support tensors do not mention it in the docstring).

Note: I kept this PR minimal and did not try to change any actual code, but the typing of timesteps in schedulers is a bit messy. For instance several schedulers initialize self.timesteps as a floating-point tensor but use an integer tensor in set_timesteps.

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Who can review?

Maybe @yiyixuxu since this is about schedulers.

[yiyixuxu]

thanks!

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