Better error message when passing `nnf_*` functions to `nn_sequential`

[dfalbel]

The current error message is not helpful at all:

Error in torch_relu(input) : argument "input" is missing, with no default

library(torch)
nn_sequential(
  nnf_relu()
)
#> Error in torch_relu(input): argument "input" is missing, with no default

^{Created on 2021-05-12 by the reprex package (v0.3.0)}