Allow dynamic batch-sizes when exporting to ONNX, instead if fixed input shapes

[BeeAlarmed]

Description

When exporting to ONNX the models input shape is fixed to a batch-size of 1. This only allows to feed single images into the resuting anomaly model.

Torch.onnx allows to define dynamic axes when exporting the model, which enables dynamic batch-sizes.

• Fixes # (issue)

Changes

Describe the changes you made

• Bug fix (non-breaking change which fixes an issue)
• Refactor (non-breaking change which refactors the code base)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Checklist

Ensure that you followed the following

• I have added a summary of my changes to the CHANGELOG (not for minor changes, docs and tests).
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas (If applicable)
• I have made corresponding changes to the documentation (If applicable)
• I have added tests that prove my fix is effective or that my feature works (If applicable)

[samet-akcay]

Thanks for your contribution!