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

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==+cpu --index-url https://download.pytorch.org/whl/cpu
```

As seen in:
- https://heroku.support/1527398
- https://heroku.support/1531439
- https://heroku.support/1531634

GUS-W-19247065.