Fix test_gradient_transfer batch_norm test case

[modiase]

Description

The test_gradient_transfer test in mo_net/tests/trainer/test_parallel.py fails for the batch_norm test case.

Current Status

Test is skipped with @pytest.mark.skip(reason="TODO: batch_norm gradient transfer broken")

Location

• File: mo_net/tests/trainer/test_parallel.py
• Test: test_gradient_transfer[1-batch_norm] and test_gradient_transfer[2-batch_norm]

Next Steps

• Investigate why gradient transfer fails specifically for BatchNorm layers
• Fix the serialization/deserialization or gradient aggregation for BatchNorm