Add torch.jit.unused to MetricCollection forward

[simran2905]

Before submitting

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

What does this PR do?

The forward function of MetricCollection has *args and **kwargs arguments. Which aren't jit supported. Add torch.jit.unused decorator.

PR review

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[codecov]

Codecov Report

Merging #307 (62c2618) into master (b22d09f) will decrease coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #307      +/-   ##
==========================================
- Coverage   96.51%   96.42%   -0.10%     
==========================================
  Files         106      106              
  Lines        3448     3444       -4     
==========================================
- Hits         3328     3321       -7     
- Misses        120      123       +3     

Flag	Coverage Δ
Linux	75.84% <100.00%> (+0.01%)	⬆️
Windows	75.84% <100.00%> (+0.01%)	⬆️
cpu	75.84% <100.00%> (-20.62%)	⬇️
gpu	96.42% <100.00%> (+<0.01%)	⬆️
macOS	75.84% <100.00%> (-20.62%)	⬇️
pytest	96.42% <100.00%> (-0.10%)	⬇️
python3.6	?
python3.8	?
python3.9	?
torch1.3.1	?
torch1.4.0	?
torch1.9.0	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
torchmetrics/collections.py	98.87% <100.00%> (+0.02%)	⬆️
torchmetrics/average.py	83.33% <0.00%> (-12.50%)	⬇️
torchmetrics/metric.py	95.43% <0.00%> (-0.08%)	⬇️
torchmetrics/utilities/prints.py	100.00% <0.00%> (ø)

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