Bugfix: reset should set clear _computed

[maximsch2]

Fixing a small bug - reset not clearing computed cache value.

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• Did you write any new necessary tests?

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[codecov]

Codecov Report

Merging #147 (9139356) into master (3d08235) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##           master     #147     +/-   ##
=========================================
  Coverage   96.76%   96.76%             
=========================================
  Files         156       78     -78     
  Lines        4824     2413   -2411     
=========================================
- Hits         4668     2335   -2333     
+ Misses        156       78     -78     

Flag	Coverage Δ
Linux	79.27% <100.00%> (+<0.01%)	⬆️
Windows	79.27% <100.00%> (+<0.01%)	⬆️
cpu	96.76% <100.00%> (+<0.01%)	⬆️
gpu	?
macOS	96.76% <100.00%> (+<0.01%)	⬆️
pytest	96.76% <100.00%> (+<0.01%)	⬆️
python3.6	96.76% <100.00%> (+<0.01%)	⬆️
python3.8	96.76% <100.00%> (+<0.01%)	⬆️
python3.9	96.76% <100.00%> (+<0.01%)	⬆️
torch1.3.1	95.55% <100.00%> (+<0.01%)	⬆️
torch1.4.0	95.69% <100.00%> (+<0.01%)	⬆️
torch1.8.1	96.76% <100.00%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
torchmetrics/metric.py	95.07% <100.00%> (+0.01%)	⬆️
__w/1/s/torchmetrics/retrieval/retrieval_metric.py
...s/torchmetrics/functional/classification/f_beta.py
...rics/functional/classification/hamming_distance.py
...1/s/torchmetrics/functional/retrieval/precision.py
__w/1/s/torchmetrics/regression/psnr.py
.../s/torchmetrics/classification/hamming_distance.py
.../1/s/torchmetrics/functional/classification/roc.py
...chmetrics/functional/classification/stat_scores.py
...1/s/torchmetrics/regression/mean_absolute_error.py
... and 69 more

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[maximsch2]

While we are on this, should we reset _forward_cache as well?

[SkafteNicki]

While we are on this, should we reset _forward_cache as well?

Fine by me :]