[hijax] prototype hijax pieces #28781
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shouldn't affect existing behaviors, or trace time
The main implementation ideas:
* each Trace is tagged with a `requires_low: bool`
* each Jaxpr
* is tagged with an `is_high: bool`, default False but set True while tracing
if any hijax primitives are encountered
* includes an `mut_types: dict[Var, HijaxType]` indicating final types for
type-changing mutable hijax types
* each AbstractValue is tagged by a `mutable: bool` which is read to populate
`mut_types`
* each Primitive
* has an `is_high(**params) -> bool` method (depends on params for HOPs)
* has a `to_lojax(*args, **params)` method taking and returning
hijaxtypes-wrapping-lowtracers
* in `Primitive.bind`, we check if `prim.is_high(**params) and
trace.requires_low`, and if so we call `prim.to_lojax`
Co-authored-by: Dougal Maclaurin <dougalm@google.com>
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This shouldn't affect any existing behaviors, and it shouldn't regress tracing time noticeably.
The main problem we're trying to solve is
grad-of-jitwhere thejitabstracts aBox. In that case, we must pass a reference to the abstracted box to the backward pass. To do that, we make it part of the AD jaxpr. And to avoid polluting all jaxprs, we introduce a new (currently AD-specific) hijax jaxpr, which has more types/primitives than the existing (lojax) jaxprs.The main implementation ideas:
requires_low: boolis_high: bool, default False but set True while tracing if any hijax primitives are encounteredmut_types: dict[Var, HijaxType]indicating final types for type-changing mutable hijax typesmutable: boolwhich is read to populatemut_typesis_high(**params) -> boolmethod (depends on params for HOPs)to_lojax(*args, **params)method taking and returning hijaxtypes-wrapping-lowtracersPrimitive.bind, we check ifprim.is_high(**params) and trace.requires_low, and if so we callprim.to_lojaxWe plan to revise the implementation to look like having a ToLojax trace that sits above the default EvalTrace instance at the top level.
Co-implemented with @dougalm. Design explanation forthcoming.