fix reshaping unet if timestep is 0d tensor

[eaidova]

What does this PR do?

during testing models I found issue with reshaping unet in diffusion pipeline. Issue connected that reshaping expect to have 1d tensor (torch.tensor([5])), while in some cases it represented as 0d tensor (torch.tensor(5)), in this case reshaping failed with

  File "/home/ea/work/py311/lib/python3.11/site-packages/optimum/intel/openvino/modeling_diffusion.py", line 661, in _reshape_unet
    shapes[inputs][0] = 1
    ~~~~~~~~~~~~~~^^^
RuntimeError: Exception from src/core/src/shape_util.cpp:65:
Accessing out-of-range dimension

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