Migrate all uses of `at::Tensor` to `torch::Tensor`.

Currently, [two files use at::Tensor](https://github.com/search?q=repo%3Apytorch%2Ftorchcodec%20at%3A%3ATensor&type=code) while the [majority of files use torch::Tensor](https://github.com/search?q=repo%3Apytorch%2Ftorchcodec+torch%3A%3ATensor&type=code
).
For consistency, we should update all uses of `at::Tensor` to `torch::Tensor`.
