Refactor mnist hello world example

[ishandutta0098]

What does this PR do?

I have refactored the entire mnist hello world example, maintaining a similar structure but much more readable.

Following are the major changes:

1. Addition of a dataclass config instead of hardcoded config values
2. Change in the implementation style of the forward() function of MNISTModel
3. Add the ckpt_path argument to trainer.test() to suppress the warning
4. Addition of docstrings to all classes and methods
5. Addition of type hints for all parameters
6. Addition of comments as and when required

Additional Information:

• I have used patched commits to ensure that the code review becomes easier.
• Code formatting has been done using black
• The code has been tested via a local run

PR review

I am a member of the Lightning League and the refactoring has been discussed with @Borda

closes #240

[codecov]

Codecov Report

Merging #241 (bf47f8e) into main (9ce4acc) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@ Coverage Diff @@ ## main #241 +/- ## =================================== Coverage 73% 73% =================================== Files 2 2 Lines 382 382 =================================== Hits 280 280 Misses 102 102