Avoid warning in tests due to torch deprecation

[NicolasHug]

Closes #748

We started getting warnings in main due to torch not allowing lists for multi-indexing anymore:

test/test_frame_dataclasses.py::test_framebatch_indexing
  /home/nicolashug/dev/torchcodec/src/torchcodec/_frame.py:107: UserWarning: Using a non-tuple sequence for multidimensional indexing is deprecated and will be changed in pytorch 2.9; use x[tuple(seq)] instead of x[seq]. In pytorch 2.9 this will be interpreted as tensor index, x[torch.tensor(seq)], which will result either in an error or a different result (Triggered internally at /pytorch/torch/csrc/autograd/python_variable_indexing.cpp:306.)
    data=self.data[key],

It's our tests we need to change, not our indexing code in frame.py. I can confirm the warnings aren't visible locally anymore (they were before).