Fix implicit conversions in VideoStreamOptions and AudioStreamOptions

[NicolasHug]

As noted in https://github.com/pytorch/torchcodec/pull/678/files#r2088653876 we have implicit conversions from std::optional<int64_t> to std::optional<int> in our *StreamOptions:

https://github.com/pytorch/torchcodec/blob/0cced7c244354d98cf0ebc81bb210901db3a8985/src/torchcodec/_core/custom_ops.cpp#L230-L232

https://github.com/pytorch/torchcodec/blob/0cced7c244354d98cf0ebc81bb210901db3a8985/src/torchcodec/_core/custom_ops.cpp#L285
(+ num_channels for audio)

We should assert the range and explicitly cast.