Delay loading the cuda library on Windows

[peterjc123]

so we can import torch compiled with cuda on a CPU-only machine.
need tests

[dr-ci]

💊 Build failures summary and remediations

As of commit 2e9f988 (more details on the Dr. CI page):

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• 2/2 failures introduced in this PR

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🕵️ 2 new failures recognized by patterns

The following build failures do not appear to be due to upstream breakages:

pytorch_linux_xenial_py3_6_gcc5_4_build (1/2)

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CONFLICT (add/add): Merge conflict in .circleci/verbatim-sources/pytorch-job-specs.yml 
Auto-merging .circleci/verbatim-sources/pytorch-job-specs.yml 
CONFLICT (add/add): Merge conflict in .circleci/verbatim-sources/job-specs-custom.yml 
Auto-merging .circleci/verbatim-sources/job-specs-custom.yml 
CONFLICT (add/add): Merge conflict in .circleci/scripts/setup_ci_environment.sh 
Auto-merging .circleci/scripts/setup_ci_environment.sh 
CONFLICT (add/add): Merge conflict in .circleci/config.yml 
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CONFLICT (add/add): Merge conflict in .circleci/cimodel/data/pytorch_build_definitions.py 
Auto-merging .circleci/cimodel/data/pytorch_build_definitions.py 
Automatic merge failed; fix conflicts and then commit the result. 

pytorch_xla_linux_bionic_py3_6_clang9_build (2/2)

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CONFLICT (add/add): Merge conflict in .circleci/verbatim-sources/pytorch-job-specs.yml 
Auto-merging .circleci/verbatim-sources/pytorch-job-specs.yml 
CONFLICT (add/add): Merge conflict in .circleci/verbatim-sources/job-specs-custom.yml 
Auto-merging .circleci/verbatim-sources/job-specs-custom.yml 
CONFLICT (add/add): Merge conflict in .circleci/scripts/setup_ci_environment.sh 
Auto-merging .circleci/scripts/setup_ci_environment.sh 
CONFLICT (add/add): Merge conflict in .circleci/config.yml 
Auto-merging .circleci/config.yml 
CONFLICT (add/add): Merge conflict in .circleci/cimodel/data/pytorch_build_definitions.py 
Auto-merging .circleci/cimodel/data/pytorch_build_definitions.py 
Automatic merge failed; fix conflicts and then commit the result. 

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This comment has been revised 9 times.

[malfet]

Aren't nvrtc designed to dynamically depend on CUDA?
It feels strange to solve it only Windows users

[peterjc123]

@malfet Symbols on Linux are resolved lazily by default. That's why importing a cuda version of torch on a CPU-only machine doesn't fail.

[ezyang]

Wish we could test this. Maybe it wouldn't be too hard to do on CircleCI? We do a similar test on Linux, loading a CUDA binary on CPU and then running tests, it has caught bugs.

[facebook-github-bot]

@ezyang is landing this pull request. If you are a Facebook employee, you can view this diff on Phabricator.

[facebook-github-bot]

@ezyang merged this pull request in 0359a9b.