Make functionalization ViewMeta serializable with pickle. (#143712)

Fix: #141974

This PR makes `ViewMeta` sequence, present in functional tensors,
serializable with pickle. In order to accomplish that, it makes
`ViewMeta` an abstract class with overridable `forward` and `reverse`
functions. In this context, each operation that once instanciated
`ViewMeta`, should now create a new specialized class that inherits from
`ViewMeta. Therefore, this PR also uses codegen for creating these
specializations.

In summary, these are the changes this PR introduces:

- `ViewMeta` is turned into an abstract class (see
  _FunctionalStorageImpl.cpp_). `forward` and `reverse` are pure virtual
  functions that need to be implemented. `to_out_index` should be
  implemented by operations that might return more than 1 output.

- New `ViewMeta` specializations for `resize_` and `_unsafe_view` are
  created (see _FunctionalizeFallbackKernel.h_).

- New templates _ViewMetaClasses.{cpp,h}_ are created. They hold the
  declaration and definition of the `ViewMeta` specializations, which
  are automatically generated in the ATen codegen (see _gen.py_).

- New `_functionalization` Python sub-module is created (see
  _Module.cpp_). It serves as namespace for the `ViewMeta`
  specializations and `InverseReturnMode` enum.

- New template _ViewMetaClassesPythonBinding.cpp_ is created. It holds
  the automatically generated Python bindings for the `ViewMeta`
  specialization, which are generated in the torch codegen (see
  _generate_code.py_).

Note that this PR makes use of codegen at 2 different moments:

- ATen codegen (_gen.py_): generates the `ViewMeta` specialized classes.
- Torch codegen (_generate_code.py_): generated the Python bindings for
  them.

Pull Request resolved: https://github.com/pytorch/pytorch/pull/143712
Approved by: https://github.com/bdhirsh

build.bzl

@@ -117,6 +117,7 @@ def define_targets(rules):
             ":LazyNonNativeIr.h",
             ":RegisterDispatchDefinitions.ini",
             ":RegisterDispatchKey.cpp",
+            ":ViewMetaClassesPythonBinding.cpp",
             ":native_functions.yaml",
             ":shape_inference.h",
             ":tags.yaml",
@@ -297,6 +298,7 @@ _GENERATED_AUTOGRAD_PYTHON_CPP = [
     "torch/csrc/autograd/generated/python_torch_functions_1.cpp",
     "torch/csrc/autograd/generated/python_torch_functions_2.cpp",
     "torch/csrc/autograd/generated/python_variable_methods.cpp",
+    "torch/csrc/functionalization/generated/ViewMetaClassesPythonBinding.cpp"
 ]
 
 GENERATED_AUTOGRAD_PYTHON = _GENERATED_AUTOGRAD_PYTHON_HEADERS + _GENERATED_AUTOGRAD_PYTHON_CPP