fix failed onnx tests

Context

This MR is intended to fix the failed onnx tests encountered in issue aidge_onnx#50 (closed) and issue aidge_onnx#46.

This includes the following problems:

• Flatten: missing python binding.
• And/Equal: A mistake was made, the operator Equal was named And.
• AvgPooling: unsupported attrs dilations and ceil_mode
• MaxPooling: unsupported attr dilations

Detailed major modifications

The following modifications were made:

• Add python binding for flatten
• Add Equal operator
• Add dilations attr to Maxpooling
• Add dilations and ceil_mode to AvgPooling

TODO

This MR would only fix the following:

• Add Flatten binding
• Add Equal Op
• AvgPooling: add dilations and ceil_mode attrs
• MaxPooling: add dilationsattr