Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/48249
Introduced autograd related data models at tools.codegen.api.autograd.
Migrated load_derivatives.py to produce the new data models from derivatives.yaml.
It has clean mypy-strict result.
Changed both gen_autograd_functions.py and gen_variable_type.py to consume
the new data model.
Added type annotations to gen_autograd_functions.py - it has clean mypy-strict
result except for the .gen_autograd import (so haven't added it to the strict
config in this PR).
To limit the scope of the PR, gen_variable_type.py is not refactored, and the
main structure of load_derivatives.py / gen_autograd_functions.py is kept. We
only make necessary changes to make it work.
Confirmed byte-for-byte compatible with the old codegen:
```
Run it before and after this PR:
.jenkins/pytorch/codegen-test.sh <baseline_output_dir>
.jenkins/pytorch/codegen-test.sh <test_output_dir>
Then run diff to compare the generated files:
diff -Naur <baseline_output_dir> <test_output_dir>
```
Test Plan: Imported from OSS
Reviewed By: ezyang
Differential Revision: D25086561
Pulled By: ljk53
fbshipit-source-id: 1f43ab0931d9814c24683b9a48ca497c5fc3d729
