Fix GradientAccumulatorDistributionStrategy test verification
Author: qxrein
Pull request: yes

# What does this PR do?
The GradientAccumulatorDistributionStrategy test wasn't properly verifying the effect of gradient application. Added assertion to check that the variable values are actually updated after applying the accumulated gradients using the optimizer with the given learning rate (5e-5). This ensures the test properly validates both gradient accumulation and application in a distributed setting.

Changes made:
1. Added proper verification of gradient application by checking the new variable values
2. Used appropriate tolerance (1e-4) for the gradient application check

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