Move assert into pipeline

[damccorm]

This assert was inadvertently ignored, until #30771 exposed that and turned the meaningless assert into an error. This broke the python postcommit - https://github.com/apache/beam/actions/workflows/beam_PostCommit_Python.yml - this fixes the problem

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 58.89%. Comparing base (818966c) to head (50dd048).

Files with missing lines	Patch %	Lines
sdks/python/apache_beam/ml/gcp/visionml_test_it.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master   #32530   +/-   ##
=========================================
  Coverage     58.89%   58.89%           
  Complexity     3071     3071           
=========================================
  Files          1129     1129           
  Lines        173950   173959    +9     
  Branches       3318     3318           
=========================================
+ Hits         102442   102452   +10     
+ Misses        68171    68170    -1     
  Partials       3337     3337           

Flag	Coverage Δ
python	81.57% <0.00%> (+<0.01%)	⬆️

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[damccorm]

R: @liferoad @Abacn

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