Calculate text color of ConfusionMatrix plot based on luminance

[relativityhd]

What does this PR do?

This will make the text-color of the ConfusionMatrix heatmap-plots white for darker-patches and white for brighter ones.
I haven't opened an Issue for this, since it is a non-intrusive and small change.

Before submitting

• Was this discussed/agreed via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
• Did you write any new necessary tests?

PR review

Assume the following Code:

import torchmetrics
import torch
pred = torch.tensor([0, 1, 2, 3, 4])
target = torch.tensor([0, 1, 2, 2, 2])
cnf = torchmetrics.ConfusionMatrix(task="multiclass", num_classes=5)
cnf.update(pred, target)
cnf.plot(cmap="Blues")

Results now in the following plot:

This is how it looked before:

Did you have fun?

Make sure you had fun coding 🙃

---

📚 Documentation preview 📚: https://torchmetrics--2590.org.readthedocs.build/en/2590/

[codecov]

Codecov Report

Attention: Patch coverage is 11.11111% with 8 lines in your changes missing coverage. Please review.

Project coverage is 69%. Comparing base (27a301a) to head (812c932).
Report is 94 commits behind head on master.

Additional details and impacted files

@@          Coverage Diff           @@
##           master   #2590   +/-   ##
======================================
- Coverage      69%     69%   -0%     
======================================
  Files         314     314           
  Lines       17702   17709    +7     
======================================
+ Hits        12199   12200    +1     
- Misses       5503    5509    +6