Unify C++ API with C++ extensions (#11510)

Summary:
Currently the C++ API and C++ extensions are effectively two different, entirely orthogonal code paths. This PR unifies the C++ API with the C++ extension API by adding an element of Python binding support to the C++ API. This means the `torch/torch.h` included by C++ extensions, which currently routes to `torch/csrc/torch.h`, can now be rerouted to `torch/csrc/api/include/torch/torch.h` -- i.e. the main C++ API header. This header then includes Python binding support conditioned on a define (`TORCH_WITH_PYTHON_BINDINGS`), *which is only passed when building a C++ extension*.

Currently stacked on top of https://github.com/pytorch/pytorch/pull/11498

Why is this useful?

1. One less codepath. In particular, there has been trouble again and again due to the two `torch/torch.h` header files and ambiguity when both ended up in the include path. This is now fixed.
2. I have found that it is quite common to want to bind a C++ API module back into Python. This could be for simple experimentation, or to have your training loop in Python but your models in C++. This PR makes this easier by adding pybind11 support to the C++ API.
3. The C++ extension API simply becomes richer by gaining access to the C++ API headers.

soumith ezyang apaszke
Pull Request resolved: https://github.com/pytorch/pytorch/pull/11510

Reviewed By: ezyang

Differential Revision: D9998835

Pulled By: goldsborough

fbshipit-source-id: 7a94b44a9d7e0377b7f1cfc99ba2060874d51535

test/cpp_extensions/cuda_extension.cpp

@@ -1,4 +1,4 @@
-#include <torch/torch.h>
+#include <torch/extension.h>
 
 // Declare the function from cuda_extension.cu. It will be compiled
 // separately with nvcc and linked with the object file of cuda_extension.cpp