BLD: create a LICENSE_BUNDLED.txt file from third_party licenses (#50745)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/50695.
Rather than maintain a LICENSE_BUNDLED.txt by hand, this build it out of the subrepos.
I ~copied and adapted the sdist handling from Numpy~ added a separate file, so the LICENSE.txt file of the repo remains in pristine condition and the GitHub website still recognizes it. If we modify the file, the website will no longer recognize the license.
This is not enough, since the license in the ~wheel~ wheel and sdist is not modified. Numpy has a [separate step](https://github.com/MacPython/numpy-wheels/blob/master/patch_code.sh) when preparing the wheel to concatenate the licenses. I am not sure where/if the [conda-forge numpy-feedstock](https://github.com/conda-forge/numpy-feedstock/) also fixes up the license.
~Should~ I ~commit~ commited the artifact to the repo and ~add~ added a test that reproducing the file is consistent.
Edit: now the file is part of the repo.
Edit: rework the mention of sdist. After this is merged another PR is needed to make the sdist and wheel ship the proper merged license.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/50745
Reviewed By: seemethere, heitorschueroff
Differential Revision: D26074974
Pulled By: walterddr
fbshipit-source-id: bacd5d6870e9dbb419a31a3e3d2fdde286ff2c94
