Fix recompute to check if data is cleared before backward without setup_recompute

[AkioHayakawa-sony]

Re-computation may perform without the necessary setup_recompute execution.

Example:

x -> F.affine -> h1 -> F.dropout -> h2 -> F.affine -> y

y.forward()
h2.data.clear()
y.backward() # grad is computed incorrectly

# Dropout::setup_recompute() must be executed before recomputation
# but not called during forward prop because h2 is cleared after forward prop.
# Therefore, h2.data is recomputed incorrectly.

This PR handles this case and raises runtime error.