Arm backend: Cleanup and enable tests in test_batch_norm.py (pytorch#21836)

* Enable tests that now pass
* Cleanup batch norm implementations


Change-Id: I9e8479399fefef2deeb8f79c43ebc82da9cf10cb


cc @digantdesai @freddan80 @per @zingo @oscarandersson8218 @mansnils
@Sebastian-Larsson @robell @rascani

Signed-off-by: Tom Allsop <tom.allsop@arm.com>

Author: tom-arm