named_parameters error when multiple references

[dfalbel]

# This returns only weight1
net$named_parameters(TRUE)
#> $weight1
#> torch_tensor
#> -2.2368
#> [ CPUFloatType{1,1} ][ requires_grad = TRUE ]
# This returns weight1 and weight2
net$named_parameters(FALSE)

This is a bug. We should only ever return unique tensors.
We could potentially make it an error to assign the same tensor to multiple names within a module to avoid having to handle this case.

Originally posted by @dfalbel in #968 (comment)