Thanks for your advices! It works! by perform backward() in model.train(), and go through forward path without updating weights of parameters, the saliency looks right.
when I input “This film is horrible!” the saliency can tell the word “horrible” is most important for prediction the emotion of this review.

The following is the updated code:
# tring Saliency now!
# for pre-processing the sentence (sentence to vectors)
def pre_process(sentence):
    tokenized = [tok.text for tok in nlp.tokenizer(sentence)]
    indexed = [TEXT.vocab.stoi[t] for t in tokenized]
    tensor = torch.LongTensor(indexed).to(device) # seq_len long tensor is important
    tensor = tensor.unsqueeze(1) # seq_len * batch_size(1)
    embedded = torch.tensor(embed(tensor),requires_grad=True) 
# here we need to get gradient for saliency computation, use requires_grad=True
    
    return embedded
# deal with input sentence
input_1 = u"This film is horrible!"
input_1 = u"This movie was sadly under-promoted but proved to be truly exceptional."
preprocess_1 = pre_process(input_1) # requires_grad = True
# we would run the model in evaluation mode

model.train() 
# if I set model.eval(), an error occur: RuntimeError: cudnn RNN backward can only be called in training mode

'''forward pass through the model to get the scores, note that RNNModel_GRU2 model doesn't perform sigmoid at the end
and we also don't need sigmoid, we need scores, so that's perfect for us.
'''
scores = model(preprocess_1)
'''
backward function on score_max performs the backward pass in the computation graph and calculates the gradient of 
score_max with respect to nodes in the computation graph
'''
scores.backward()
'''
Saliency would be the gradient with respect to the input now. 
But note that the input has 100 dim embdeddings. 
To derive a single class saliency value for each word (i, j),  
we take the maximum magnitude across all embedding dimensions.
'''
saliency, _ = torch.max(preprocess_1.grad.data.abs(),dim=2) # AttributeError: 'NoneType' object has no attribute 'data'