Rename DefaultBackend to CompositeExplicitAutograd (#54470)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/54470
```
git grep -l 'DefaultBackend' | xargs sed -i 's/DefaultBackend/CompositeExplicitAutograd/g'
```
Plus a quick fixup in native/README.md
Signed-off-by: Edward Z. Yang <ezyang@fb.com>
Test Plan: Imported from OSS
Reviewed By: bdhirsh
Differential Revision: D27253240
Pulled By: ezyang
fbshipit-source-id: 964df951ea8b52fa72937f3cc66aeaf49a702e6f
