Improve code coverage

Our project code coverage start to decrease with new features and reworked tests. We are now ~95% but were 96% recently and around ~98% the last year...

The idea is to identify the uncovered parts of code inspecting here, for example: https://codecov.io/gh/pytorch/ignite/tree/master/ignite and draft a PR with new tests.

Some of new test can be very simple and others could be very sophisticated to enter a particular code line. Let's improve the coverage of all modules, except distributed for instance.

• ignite.base
• ignite.contrib.engines
• ignite.contrib.handlers
• ignite.contrib.metrics
• ignite/engine/__init__.py
• ignite/engine/engine.py
• ignite/engine/events.py
• ignite/handlers/checkpoint.py
• ignite.metrics