Add Batch info for export

Context

Adding missing batch info of tensor in node_export for some export kernel in export_cpp (such as Pad2D or Batchnorm2D)

Used by MR aidge_export_cpp!36 (merged)

Modified files

(Summarize the changes you made for each modified file - example:

• node_export.py to add a get_batch function that output the size of the tensor's batch;