Move checking in r2 for number of samples to compute

[SkafteNicki]

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What does this PR do?

Fixes #423
R2 score can only be calculated when having two samples. We check for this, but the check should first happen in the _compute method instead of the _update method. This PR moves the check.

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[codecov]

Codecov Report

Merging #426 (9e40f98) into master (ad9845b) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #426   +/-   ##
=======================================
  Coverage   96.08%   96.08%           
=======================================
  Files         126      126           
  Lines        4088     4088           
=======================================
  Hits         3928     3928           
  Misses        160      160           

Flag	Coverage Δ
Linux	74.68% <50.00%> (ø)
Windows	74.68% <50.00%> (ø)
cpu	96.01% <100.00%> (ø)
gpu	96.01% <100.00%> (ø)
macOS	96.01% <100.00%> (ø)
pytest	96.08% <100.00%> (ø)
python3.6	95.25% <100.00%> (ø)
python3.8	96.01% <100.00%> (ø)
python3.9	95.91% <100.00%> (ø)
torch1.3.1	95.25% <100.00%> (ø)
torch1.4.0	95.32% <100.00%> (ø)
torch1.9.0	95.91% <100.00%> (ø)

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Impacted Files	Coverage Δ
torchmetrics/functional/regression/r2.py	95.12% <100.00%> (ø)

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[tchaton]

LGTM !