Docs: fix for R^2 and SMAPE

[profPlum]

📚 Documentation

SMAPE docs say: "smape [...] A tensor with non-negative floating point smape value between 0 and 1"
R^2 docs say: "By definition the score is bounded between 0 and 1"

Both of these are incorrect:

• SMAPE ranges between [0,2]
• R^2 ranges between (-∞,1]
  • This is true when generalized beyond linear models, as is the case for torchmetrics.
  • But tbh it might be helpful to add an optional flag that can clip the range to [-1,1], for the sake of tensorboard readability? I'll leave this decision to you.

P.S.
Here is an example tensorboard plot that is hard to read due to exploding negative R^2:

[Borda]

Thank you @profPlum mind sending a PR to fix this issue? 🐰
cc: @SkafteNicki