How to change parameters of pre-trained longformer model from huggingface

I am using Hugging-face pre-trained LongformerModel model. I am using to extract embedding for sentence. I want to change the token length, max sentence length parameter but I am not able to do so. Here is the code.

model = LongformerModel.from_pretrained('allenai/longformer-base-4096',output_hidden_states = True)
tokenizer = LongformerTokenizer.from_pretrained('allenai/longformer-base-4096')


text=[" I like to play cricket"]

input_ids = torch.tensor(tokenizer.encode(text,max_length=20,padding=True,add_special_tokens=True)).unsqueeze(0)


# [0, 38, 101, 7, 310, 5630, 2]

I expected encoder to give me list of size 20 with padding as I have passed a parameter max_length=20. But it returned list of size 7 only?

attention_mask = torch.ones(input_ids.shape, dtype=torch.long, device=input_ids.device)
attention_mask[:, [0,-1]] = 2
outputs = model(input_ids, attention_mask=attention_mask, return_dict=True)

hidden_states = outputs[2]

print ("Number of layers:", len(hidden_states), "  (initial embeddings + 12 BERT layers)")
        layer_i = 0

print ("Number of batches:", len(hidden_states[layer_i]))
        batch_i = 0

print ("Number of tokens:", len(hidden_states[layer_i][batch_i]))
        token_i = 0

print ("Number of hidden units:", len(hidden_states[layer_i][batch_i][token_i]))


Number of layers: 13   (initial embeddings + 12 BERT layers)
Number of batches: 1
Number of tokens: 512 # How can I change this parameter to pick up my sentence length during run-time
Number of hidden units: 768

How can I reduce number of tokens to sentence length instead of 512 ? Every-time I input a new sentence, it should pick up that length.