Missing Conv2D and AveragePooling2D modules in export/import to NIR

[SirineArfa]

Description

The currently supported modules in the snntorch's export_nir.py are [Leaky, Linear, Synaptic, RLeaky, RSynaptic]. In my case I need to convert an SNN model with Conv2D, Leaky, Linear and AveragePooling2D layers to NIR.

What could be done

Two possible options to solve this:

1. Naively add these two torch.nn layer operations to the snntorch/export_nir.py function
2. Centralizing NIRTorch functions from_nir.py and to_nir.py to support the NIR conversion of torch.nn layers regardless of the used framework (snntorch,norse,sinabs...)