support dim==3 tensors for node export (format NCW)

Context

I've got another incoming Merge Request to implement Conv1D operator CPP export : aidge_export_cpp!52 (closed)

For that I needed node_export.py to support tensors of dimension 3 (format NCW or NWC).

Modified files

• node_export.py getters implemented for channels / height(==1) / width / batch size for a given tensor of dim 3 (format NCW or NWC)