Removing one of the twice defined position_embeddings in LongFormer (#23343)

Removing twice defined position_embeddings

The self.position_embeddings in LongFormerEmbeddings is defined twice.
Removing the first with padding_idx

src/transformers/models/longformer/modeling_longformer.py

@@ -438,7 +438,6 @@ class LongformerEmbeddings(nn.Module):
     def __init__(self, config):
         super().__init__()
         self.word_embeddings = nn.Embedding(config.vocab_size, config.hidden_size, padding_idx=config.pad_token_id)
-        self.position_embeddings = nn.Embedding(config.max_position_embeddings, config.hidden_size)
         self.token_type_embeddings = nn.Embedding(config.type_vocab_size, config.hidden_size)
 
         # self.LayerNorm is not snake-cased to stick with TensorFlow model variable name and be able to load