Add warning when using the `torch` GPU package instead of the CPU variant

[edmorley]

Similar to #1343, but for PyTorch.

We should add a warning when we see that an app has installed the torch package from PyPI, which includes all of the GPU packages (that won't actually be used on Heroku) and results in a multi-gigabyte slug size.

PyTorch does have a CPU-only package variant, but it needs the correct package index to be specified. See:
https://pytorch.org/get-started/locally/

eg something like:

torch==<version>+cpu --index-url https://download.pytorch.org/whl/cpu

As seen in:

• https://heroku.support/1527398
• https://heroku.support/1531439
• https://heroku.support/1531634