Set self.fitted = True instead of self._fitted.

[topskychen]

All the current models are using fitted to indicate the model is fitted. Current _fitted should be a bug due to legacy code.

Description

Use self.fitted = True instead of self._fitted = True in pytorch_nn.py.

Motivation and Context

How Has This Been Tested?

• Pass the test by running: pytest qlib/tests/test_all_pipeline.py under upper directory of qlib.
• If you are adding a new feature, test on your own test scripts.

Screenshots of Test Results (if appropriate):

1. Pipeline test:
2. Your own tests:

Types of changes

• Fix bugs
• Add new feature
• Update documentation

[you-n-g]

@topskychen Thanks so much for fixing the bug!