Pyglove

* Highlights:
- All PyGlove core modules are split into smaller files for better readability and extensibility. ([775470f](https://github.com/google/pyglove/commit/775470f645a7f7eb4b5f418f8b213dab207ca0a1), [2561dc4](https://github.com/google/pyglove/commit/2561dc41754ba40446030f151d21ad7f845186da), [67e84](https://github.com/google/pyglove/commit/67e8426a0e0c8676aaa03e44151d7de6eec5cdf5), [b210121](https://github.com/google/pyglove/commit/b2101214cd1f376b0b9eb3122de567a91b951ff3), [cb244a](https://github.com/google/pyglove/commit/cb244a723bf978d1c38acdacee4a3c111751a6e7), [5026b3d](https://github.com/google/pyglove/commit/5026b3d604372109c3e3ff8f8e0528efb369f831), [9603443](https://github.com/google/pyglove/commit/96034438482c630891279799f2940ee0f0f42614), [3cbb66e](https://github.com/google/pyglove/commit/3cbb66e9ce2fbe8598b7abe96b07c1d38b890eb7))
- Non-core PyGlove modules now reside in `pyglove/ext`. ([4994398](https://github.com/google/pyglove/commit/49943983885c8b545139807cf37c23a37f7caa4f))

* API changes
- Removes non-DNA generators symbols from `pg.generators` sub-module. ([4605a97](https://github.com/google/pyglove/commit/4605a9739eb500bce226ab8839bbebafed077231))
- Simplifies tuning backend interface by merging `pg.tuning.BackendFactory` into `pg.tuning.Backend`. ([7e56bb7](https://github.com/google/pyglove/commit/7e56bb72e42c7bd016b6370d251d93a79af70a0d))
- Renames `pg.patching.object_factory` to `pg.patching.ObjectFactory`. ([2d65a16](https://github.com/google/pyglove/commit/2d65a166437de1869809fbe78eccda504a5749f4))

* Enhancement
- Supports `pg.random_dna` and `pg.iter` for `pg.hyper.CustomHper`. ([49fc46a](https://github.com/google/pyglove/commit/49fc46a158a5f50b4e4dd121c8353a8ddc3d42fe))

* Bug fixes
- `pg.evolution.Evolution`: Excludes infeasible trials from initial population during recovery. ([d55f1dc](https://github.com/google/pyglove/commit/d55f1dca2a5deebfa4ad470cf74154ff2f7fda77))

* New features
- Introduces symbolic comparison: `pg.lt` and `pg.gt`. ([747d6a](https://github.com/google/pyglove/commit/747d6a80280a342ef39034a33d86d05be06f4520))
- Introduces `pg.evolve` for evolving arbitrary symbolic objects. ([f2606eb](https://github.com/google/pyglove/commit/f2606eb6fb8155e749a1a6195c6aff319065500c))

Enhancements:

- `pg.symbolic`:
- Introducing `pg.Symbolic.sym_has` and `pg.Symbolic.sym_get` to test/get node in sub-tree by path.
- Introducing `pg.Symbolic.sym_field` and `pg.Symbolic.sym_attr_field` to access the `pg.typing.Field` definition for current/child symbolic value.
- `pg.Symbolic.sym_getattr` now supports default value.
- `pg.Symbolic.rebind` now supports `pg.KeyPath` objects as keys.

Bug fixes:

- `pg.tuning`: Fix `pg.tuning.Feedback.should_stop_early` crash on in-memory tuning backend.

New features:

- (experimental) Introducing `pg.composing` to handle algorithmic symbolic mutations.

* New Features
- Introduce `pg.logging` to allow pluggable PyGlove logging.
- Introduce `pg.early_stopping` with initial step-wise policies.

* Enhancements
- `pg.hyper`:
- Support nested `pg.hyper.DynamicEvaluationContext` for supporting divide-and-conquer a search space.
- Support scale type for `pg.floatv`.
- Preserve numeric literal values when calling `pg.hyper.Choices.dna_spec`.

- `pg.tuning`:
- Support using base error class and error regex in `pg.tuning.Feedback.skip_on_exceptions`.
- Add `pg.tuning.Feedback.ignore_race_conditions` to better handle race conditions from multiple co-workers

- `pg.evolution`:
- Make `pg.evolution.Evolution` thread-safe during `propose`/`feedback`.
- Make `pg.evolution.Evolution` better report population initialization errors.