Check if torch is installed on tests

[manoelmarques]

Summary

Fixes #54

Details and comments

[drewrisinger]

• Diff LGTM
• CI Tests pass

Looks like your solution was to wrap the test in a try/catch block. I was wondering if a fixture something like https://github.com/Qiskit/qiskit-terra/blob/0fc83f44a6e80969875c738b2cee7bc33223e45f/qiskit/test/decorators.py#L66-L76
would be a better way to do it, but that could be a later step.

[woodsp-ibm]

I noticed, via the CI change to uninstall what we install as extras, that we torchvision as an extra requires. From what I can tell torchvision is only used in the notebook tutorial, not in the core library code. But if the user installs torch via the pip install qiskit-machine-learning[torch] they will get torchvision that the library does not use/need. I think the extra requires should be limited to what the library needs/can use i.e. torch and should not install torchvision that maybe a user does not want and certainly does not need for the core library. We can install torchvision in the CI for running notebooks (or any other library that the notebook examples may use in the future that the core does include/need).

[manoelmarques]

I removed torchvision from extra requires.