Refactor C++ ops implementations

[scotts]

This PR refactors two things:

1. Removes VideoDecoderOps.h, the header file that declared all of the C++ custom ops. We actually don't need this anymore, as the purpose it served was to enable other C++ code to call the C++ custom ops. But we deleted the C++ test that did that, and now these functions are only ever called through their Python bindings. Removing this file simplifies our code and simplifies the process of adding a new custom op, which already has a bunch of steps. The comments in the header were valuable, so I moved them over.
2. Renames the .cpp files that are the main logic for libtorchdoec_custom_ops.so and libtorchcodec_pybind_ops.so to match: custom_ops.cpp and pybind_ops.cpp.