Prevent users from overwriting non-existent fields

[frostedoyster]

Using metatensor-models, it's easy to try to override something in a wrong way.
e.g.

model:
  layernorm: false

when in reality the correct syntax is

model:
  bpnn:
    layernorm: false

In that case, no error is raised, the user thinks they're running without layernorms, but the layernorms are still on