lazy load torch

[DavidLapous]

I noticed a significant increase in the loading time of my library (multipers) and tracing back the imports I ended up here.

The current issue is that if torch_found in the __init__.py, then the line

    from .torch.test_install import test_torch_bindings

will trigger the pykeops/torch/__init__.py, and load torch.

If we want to have a lazy load of torch, these functions thus need to be put outside this torch folder.

In this PR, I propose to do a new hidden module _test_install, and put the test_{stuff}_bindings here.
I also put the import torch inside the function to avoid importing it.