You could use copy_ instead of fill_ or assign an nn.Parameter.
Also, the usage of the .data attribute is discouraged. Wrap the assignment in a torch.no_grad() block instead:
numpy_data= np.random.randn(6, 1, 3, 3)

conv = nn.Conv2d(1, 6, 3, 1, 1, bias=False)
with torch.no_grad():
    conv.weight = nn.Parameter(torch.from_numpy(numpy_data).float())
    # or
    conv.weight.copy_(torch.from_numpy(numpy_data).float())