.github: Improve sanity check for generated files (#86143)
Makes it so that generated files from .gitattributes do not affect the
pr-sanity-check
Tested using: (https://github.com/pytorch/pytorch/pull/86143)
```
❯ BASE=d401732baadf2df666f242cd32db5df3b09dbec6 HEAD=eaf9aa24acf6a1fc68243935f4b33188a59bfdd2 bash .github/scripts/pr-sanity-check.sh
INFO: Checking aginst the following stats
+ git diff --stat 6d06be89fe2b9ca30c3d97475dd192fc7e3f7357 eaf9aa24acf6a1fc68243935f4b33188a59bfdd2
+ sed '$d'
INFO: Showing non-generated files:
+ cat /tmp/tmp.mQeK24emtZ
.github/scripts/test_trymerge.py | 2 +-
.github/scripts/trymerge.py | 14 +
INFO: PR SIZE is 16
```
Signed-off-by: Eli Uriegas <eliuriegas@fb.com>
Pull Request resolved: https://github.com/pytorch/pytorch/pull/86143
Approved by: https://github.com/malfet, https://github.com/albanD
