Bump version to 0.5.1rc0

[SeanNaren]

What does this PR do?

Bumps the version to create an RC for the next release!

[codecov]

Codecov Report

Merging #850 (fca6e13) into master (b05a121) will decrease coverage by 13.13%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           master     #850       +/-   ##
===========================================
- Coverage   84.85%   71.72%   -13.14%     
===========================================
  Files         224      224               
  Lines       12117    12117               
===========================================
- Hits        10282     8691     -1591     
- Misses       1835     3426     +1591     

Flag	Coverage Δ
unittests	71.72% <100.00%> (-13.14%)	⬇️

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

Impacted Files	Coverage Δ
flash/__about__.py	100.00% <100.00%> (ø)
flash/core/integrations/icevision/transforms.py	13.33% <0.00%> (-80.84%)	⬇️
flash/core/serve/dag/visualize.py	19.04% <0.00%> (-76.20%)	⬇️
flash/core/serve/component.py	21.69% <0.00%> (-72.65%)	⬇️
...lash/image/embedding/vissl/transforms/multicrop.py	27.27% <0.00%> (-69.70%)	⬇️
...lash/image/embedding/vissl/transforms/utilities.py	14.63% <0.00%> (-68.30%)	⬇️
flash/image/detection/serialization.py	28.57% <0.00%> (-65.31%)	⬇️
flash/image/embedding/heads/vissl_heads.py	36.36% <0.00%> (-63.64%)	⬇️
flash/core/serve/interfaces/http.py	35.44% <0.00%> (-62.03%)	⬇️
flash/core/serve/flash_components.py	29.62% <0.00%> (-61.12%)	⬇️
... and 68 more

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