model.load_state_dict() does not return the model, but the information about incompatible keys, so you should remove the assignment and rerun the code:
model_params = {'weight_matrix': None,
'output_size': 1,
'hidden_dim': 512,
'n_layers': 2,
'dropout_prob': 0.001,
'vocab_size' :1259,
'embedding_dim': 300
}
model = SentimentNet(**model_params)
model.load_state_dict(torch.load('state_dict.pt', map_location=device))

model.eval()

PS: you can add code snippets by wrapping them into three backticks ```, which makes debugging easier