Fixes to parsing of pytorch models when using torch functionals

[JanFSchulte]

A# Description

When torch functionals are used for pooling and flatten operations, some arguments were not correctly caught (or in the case of flatten, this type of usage was not supported at all). This PR fixes these issues.

Type of change

• Bug fix (non-breaking change that fixes an issue)

Tests

The example model in https://github.com/pytorch/examples/blob/main/mnist/main.py would fail to parse before, and now succeeds, modulo the not supported log_softmax activiation in the end.

Checklist

• I have read the guidelines for contributing.
• I have commented my code, particularly in hard-to-understand areas.
• My changes generate no new warnings.
• I have installed and run pre-commit on the files I edited or added.
• I have added tests that prove my fix is effective or that my feature works.