Update StopWatch to have a context manager and simplify slightly

[eddiebergman]

This PR updates the Stopwatch used in automl.py and smbo.py mainly. It seems like it was previously used for timing certain steps.

Main update is that StopWatch has a context manager and automl.py::AutoML::fit now uses the context manager to help break up the different steps occuring.

from autosklearn.util.stopwatch import StopWatch

sw = StopWatch()

with sw.time("My Task Name") as task:
    # ... do stuff
    
print(f"{task.name} took {task.wall_duration}")

[codecov]

Codecov Report

Merging #1425 (1a6a23a) into development (23bfdf8) will decrease coverage by 0.15%.
The diff coverage is 69.14%.

@@               Coverage Diff               @@
##           development    #1425      +/-   ##
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- Coverage        84.46%   84.31%   -0.16%     
===============================================
  Files              147      147              
  Lines            11293    11284       -9     
  Branches          1924     1934      +10     
===============================================
- Hits              9539     9514      -25     
- Misses            1245     1255      +10     
- Partials           509      515       +6