Prevent TensorboardLogger from blocking the process exit

[lrzpellegrini]

When using __del__ as a finalizer, the TensorboardLogger could not properly free the SummaryWriter thus blocking the process exit.

The new finalization logic uses finalize (official example) to cleanup the writer object at process shutdown (or at logger garbage collection time).

Closes #864

[ContinualAI-bot]

Oh no! It seems there are some PEP8 errors! 😕
Don't worry, you can fix them! 💪
Here's a report about the errors and where you can find them:

avalanche/logging/tensorboard_logger.py:68:81: E501 line too long (99 > 80 characters)
1       E501 line too long (99 > 80 characters)

[coveralls]

Pull Request Test Coverage Report for Build 1619418330

• 1 of 2 (50.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 79.826%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
avalanche/logging/tensorboard_logger.py	1	2	50.0%

Totals
Change from base Build 1619243472:	0.0%
Covered Lines:	11451
Relevant Lines:	14345

---

💛 - Coveralls