title: ERROR:optimizer got an empty parameter list
body: <p>Here is code:</p>
  <pre><code>import torch
  import matplotlib.pyplot as plt
  from sklearn.manifold import TSNE
  from torch_geometric.datasets import Planetoid
  from torch_geometric.nn import Node2Vec
  from collections import namedtuple
  
  dataset = Planetoid(root='tmp/', name='Cora')
  data = dataset[0]
  
  device = 'cuda' if torch.cuda.is_available() else 'cpu'
  model = Node2Vec(data.edge_index, embedding_dim=128, walk_length=20,
                   context_size=10, walks_per_node=10, num_negative_samples=1,
                   sparse=True).to(device)
  loader = model.loader(batch_size=128, shuffle=True, num_workers=4)
  
  optimizer = torch.optim.SparseAdam(model.parameters(), lr=0.01)
  </code></pre>
  <p>when running these codes</p>
  <pre><code>optimizer = torch.optim.SparseAdam(model.parameters(), lr=0.01)
  </code></pre>
  <p>I got:</p>
  <pre><code>ValueError: optimizer got an empty parameter list
  </code></pre>
  <p>I am so confused and I can't solve the problem. Thanks for your help!</p>