init is too restrictive for svd or knn methods

[rofinn]

I think if all Imputors or Chains were just callable then I think the init field for svd could just be a callable with 3 assumptions that can be explicitly specified.

1. the init callable should fill in all values
2. it does not mutate the original data
3. follow the method signature of init(data; dims=dims)

If any of those fail then it can just error appropriately at runtime either with a method error or explicit assertion.

Related to #89