[Task]: Make Batch Dimension dynamic for ONNX export

[Marcus1506]

What is the motivation for this task?

1. Currently when specifying the input shape when using Engine.export to ONNX, the standard behavior is to fix the batch dimension. I don't think this reflects normal usecases for practitioner's really well and it is somewhat inconsistent for the following reason:
2. When specifying no input shape, the batch size as well as height and width are treated as dynamic. I think when specifying input shape, it would make sense to have batch size dynamic, as there is currently no way of specifying a fixed batch size.

Describe the solution you'd like

1. One solution would be to make batch size dynamic when specifying input shape in ExportMixin.to_onnx.
2. Another solution, to perhaps also retain the functionality of fixed batch sizes, could be to add an additional argument to the export function.

Additional context

No response