[Doc] Missing documentation about how ITensor and IRWTensor works as an interop interface to pytorch

[jkwak-work]

One of the slangpy + pytorch examples, "soft-rasterizer-pytorch", uses an interface ITensor.
ITensor in the example plays an important role as an interop interface to pytorch.

But we currently don't have any document or even any comments on the source code about how ITensor works as an interop to pytorch.

The implementation can be found here,
https://github.com/shader-slang/slangpy/blob/main/slangpy/slang/tensor.slang

A good place for the information will be in "PyTorch" section in the slangpy document,
https://slangpy.shader-slang.org/en/latest/src/autodiff/pytorch.html

And we should also have some documentation in the API reference as well.
https://slangpy.shader-slang.org/en/latest/src/api_reference.html

[jkwak-work]

Assigning to myself until we can figure out who can work on this.