Allow Visualizers to plot predictions without ground truth

[AdeelH]

Overview

This PR allows Visualizers to plot predictions without ground truth. That is, one can now do:

viz.plot_batch(x, y=None, z=z)

Checklist

• Added unit tests, if applicable
• Updated documentation, if applicable
• Added needs-backport label if the change should be back-ported to the previous release
• PR has a name that won't get you publicly shamed for vagueness

Notes

N/A

Testing Instructions

See new unit tests.

[codecov]

Codecov Report

Merging #1987 (c4534ae) into master (cd102da) will increase coverage by 0.04%.
Report is 9 commits behind head on master.
The diff coverage is 90.35%.

@@            Coverage Diff             @@
##           master    #1987      +/-   ##
==========================================
+ Coverage   86.14%   86.19%   +0.04%     
==========================================
  Files         195      195              
  Lines        9581     9624      +43     
==========================================
+ Hits         8254     8295      +41     
- Misses       1327     1329       +2     

Files	Coverage Δ
...s_sagemaker/rastervision/aws_sagemaker/__init__.py	100.00% <ø> (ø)
...vision_core/rastervision/core/analyzer/analyzer.py	100.00% <ø> (ø)
...ervision_core/rastervision/core/backend/backend.py	100.00% <ø> (ø)
...ision/core/data/crs_transformer/crs_transformer.py	100.00% <ø> (ø)
...vision_core/rastervision/core/data/label/labels.py	100.00% <ø> (ø)
...on/core/data/label/semantic_segmentation_labels.py	91.53% <ø> (ø)
...astervision/core/data/label_source/label_source.py	78.57% <100.00%> (+3.57%)	⬆️
.../rastervision/core/data/label_store/label_store.py	100.00% <100.00%> (ø)
...a/label_store/semantic_segmentation_label_store.py	94.41% <100.00%> (ø)
...tervision/core/data/raster_source/raster_source.py	91.30% <100.00%> (+0.25%)	⬆️
... and 20 more