Inside of forward function of  class LongformerSelfAttention(nn.Module) 1:

The attention_mask is changed in BertModel.forward from 0, 1, 2 to
-ve: no attention
0: local attention
+ve: global attention

I wonder how this change happens.

Longformer inherits from RobertaModel, which inherits from BertModel. Would BertModel’s forward also be called when calling Longformer 's forward function?
How BertModel’s forward change 0, 1, 2 to -ve/0/+ve？

Thank you