Dan Zheng 146c11ec80 [AutoDiff upstream] Add differentiable_function canonicalization. (#30818) ...

Canonicalizes `differentiable_function` instructions by filling in missing
derivative function operands.

Derivative function emission rules, based on the original function value:

- `function_ref`: look up differentiability witness with the exact or a minimal
  superset derivative configuration. Emit a `differentiability_witness_function`
  for the derivative function.
- `witness_method`: emit a `witness_method` with the minimal superset derivative
  configuration for the derivative function.
- `class_method`: emit a `class_method` with the minimal superset derivative
  configuration for the derivative function.

If an *actual* emitted derivative function has a superset derivative
configuration versus the *desired* derivative configuration, create a "subset
parameters thunk" to thunk the actual derivative to the desired type.

For `differentiable_function` instructions formed from curry thunk applications:
clone the curry thunk (with type `(Self) -> (T, ...) -> U`) and create a new
version with type `(Self) -> @differentiable (T, ...) -> U`.

Progress towards TF-1211.

2020-04-05 20:19:10 -07:00

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