in_feature is the number of inputs for your linear layer:
# constructor of nn.Lienar
def __init__(self, in_features, out_features, bias=True):
    super(Linear, self).__init__()
    self.in_features = in_features # num inputs
    self.out_features = out_features # num outputs
    self.weight = Parameter(torch.Tensor(out_features, in_features))
    if bias:
        self.bias = Parameter(torch.Tensor(out_features))

You can still check in the sources: https://github.com/pytorch/pytorch/blob/master/torch/nn/modules/linear.py 1.3k