Determine num_channels and dtype in RasterSource statically without reading a chip

[AdeelH]

Overview

The num_channels and dtype of a RasterSource can be different from the underlying data source because of RasterTransformers. Previously, these could only be determined dynamically at runtime by reading a 1x1 chip and checking its shape and dtype. This can be very slow when initializing many RasterSources or when initializing a RasterSources with many bands.

This PR adds get_out_channels() and get_out_dtype() methods to RasterTransformers, allowing num_channels and dtype to be determined statically without reading any data.

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/updated unit tests.

[codecov]

Codecov Report

Attention: Patch coverage is 98.07692% with 1 line in your changes missing coverage. Please review.

Project coverage is 90.34%. Comparing base (b263422) to head (d8094ea).
Report is 29 commits behind head on master.

Files with missing lines	Patch %	Lines
...e/data/raster_source/multi_raster_source_config.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2212      +/-   ##
==========================================
+ Coverage   90.21%   90.34%   +0.12%     
==========================================
  Files         198      199       +1     
  Lines        9906     9908       +2     
==========================================
+ Hits         8937     8951      +14     
+ Misses        969      957      -12     

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