Add repr to TruncatedDiagonalMVN class

[aakashdp6548]

torch.Distribution doesn't have a default __repr__ apparently, so trying to print TruncatedDiagonalMVN fails with NotImplementedError. This PR just adds a simple __repr__ that prints it similar to what it used to be before:

>>> print(pred["loc"])
TruncatedDiagonalMVN(Normal(loc: torch.Size([1, 18, 18, 2]), scale: torch.Size([1, 18, 18, 2])))

[codecov]

Codecov Report

Merging #886 (5f9dd85) into master (4d829b3) will decrease coverage by 0.05%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #886      +/-   ##
==========================================
- Coverage   95.58%   95.54%   -0.05%     
==========================================
  Files          21       21              
  Lines        2264     2266       +2     
==========================================
+ Hits         2164     2165       +1     
- Misses        100      101       +1     

Flag	Coverage Δ
unittests	95.54% <50.00%> (-0.05%)	⬇️

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Impacted Files	Coverage Δ
bliss/unconstrained_dists.py	87.30% <50.00%> (-1.23%)	⬇️

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