Speedup render_junit (#57641)
Summary:
JUnitXml.__iadd__() is very slow
But since testsuites are flattened anyway in
`convert_junit_to_testcases` concatenate flattened tests right away
As result, parsing test-reports folder with 393 files and 25+ test cases
takes .5 sec instead of 193 sec
Fix typing errors and add script to mypy-strict
Print warning, rather than abort if xml can not be parsed
Pull Request resolved: https://github.com/pytorch/pytorch/pull/57641
Reviewed By: samestep
Differential Revision: D28224401
Pulled By: malfet
fbshipit-source-id: 3efc079c1c0deef8fff5ddf083268885b28418f9
