feat: enable jax backend for virtual arrays

[ikrommyd]

Title explains it. This is a PR we'll need to revert if the jax backend gets dropped one day.

[ikrommyd]

I've ran the full suite with the jax tests enabled in the https://github.com/scikit-hep/awkward/tree/ikrommyd/test-virtual-arrays branch and I'm only getting these:

FAILED tests/test_1490_jax_reducers_combinations.py::test_bool_raises[all-0] - ValueError: not enough values to unpack (expected 3, got 0)
FAILED tests/test_1762_jax_behavior_support.py::test_jvp_nested_list - ValueError: not enough values to unpack (expected 3, got 0)
FAILED tests/test_1447_jax_autodiff_slices_ufuncs.py::test_numpyarray_grad_3 - ValueError: not enough values to unpack (expected 3, got 0)
FAILED tests/test_1447_jax_autodiff_slices_ufuncs.py::test_regular_array_3 - ValueError: not enough values to unpack (expected 3, got 0)
FAILED tests/test_1447_jax_autodiff_slices_ufuncs.py::test_regular_array_4 - ValueError: not enough values to unpack (expected 3, got 0)
FAILED tests/test_2638_mean_and_count_grads.py::test - ValueError: not enough values to unpack (expected 3, got 0)
FAILED tests/test_1447_jax_autodiff_slices_ufuncs.py::test_recordarray_7 - ValueError: not enough values to unpack (expected 3, got 0)
FAILED tests/test_1490_jax_reducers_combinations.py::test_reducer[sum-0] - ValueError: not enough values to unpack (expected 3, got 0)
FAILED tests/test_1490_jax_reducers_combinations.py::test_reducer[prod0-0] - ValueError: not enough values to unpack (expected 3, got 0)
FAILED tests/test_1490_jax_reducers_combinations.py::test_reducer[min-0] - ValueError: not enough values to unpack (expected 3, got 0)
FAILED tests/test_1490_jax_reducers_combinations.py::test_reducer[max-0] - ValueError: not enough values to unpack (expected 3, got 0)
FAILED tests/test_1490_jax_reducers_combinations.py::test_reducer[mean-0] - ValueError: not enough values to unpack (expected 3, got 0)
FAILED tests/test_1490_jax_reducers_combinations.py::test_reducer[mean-1] - ValueError: not enough values to unpack (expected 3, got 0)
FAILED tests/test_1490_jax_reducers_combinations.py::test_reducer[mean-None] - ValueError: not enough values to unpack (expected 3, got 0)
FAILED tests/test_1490_jax_reducers_combinations.py::test_reducer[prod1-0] - ValueError: not enough values to unpack (expected 3, got 0)
FAILED tests/test_1490_jax_reducers_combinations.py::test_reducer[ptp-0] - ValueError: not enough values to unpack (expected 3, got 0)
FAILED tests/test_1490_jax_reducers_combinations.py::test_reducer[ptp-1] - ValueError: not enough values to unpack (expected 3, got 0)
FAILED tests/test_1490_jax_reducers_combinations.py::test_reducer[ptp-None] - ValueError: not enough values to unpack (expected 3, got 0)
FAILED tests/test_1490_jax_reducers_combinations.py::test_reducer[std-0] - ValueError: not enough values to unpack (expected 3, got 0)
FAILED tests/test_1490_jax_reducers_combinations.py::test_reducer[std-1] - ValueError: not enough values to unpack (expected 3, got 0)
FAILED tests/test_1490_jax_reducers_combinations.py::test_reducer[std-None] - ValueError: not enough values to unpack (expected 3, got 0)
FAILED tests/test_1490_jax_reducers_combinations.py::test_int_output_reducer[argmin-0] - ValueError: not enough values to unpack (expected 3, got 0)
FAILED tests/test_1490_jax_reducers_combinations.py::test_int_output_reducer[argmax-0] - ValueError: not enough values to unpack (expected 3, got 0)
FAILED tests/test_1490_jax_reducers_combinations.py::test_sort[sort-0] - ValueError: not enough values to unpack (expected 3, got 0)
FAILED tests/test_1490_jax_reducers_combinations.py::test_bool_raises[any-0] - ValueError: not enough values to unpack (expected 3, got 0)

I've checked locally and this is an artifact of the wrapping inside the Index and the NumpyArray that we do. Didn't try to understand why that happens yet due to the wrapping but I've checked the such tests pass locally with TRUE virtual arrays. For example:

import awkward as ak
ak.jax.register_and_check()
import numpy as np
import jax.numpy as jnp
import jax as jaxlib

from awkward._nplikes.virtual import VirtualArray, materialize_if_virtual
from awkward._nplikes.numpy import Numpy
from awkward._nplikes.jax import Jax
from awkward._nplikes.shape import unknown_length

numpy = Numpy.instance()
jax = Jax.instance()

def generator_offsets():
    print("CALLING OFFSETS GENERATOR!")
    return np.array([0, 3, 3, 5], dtype=np.int64)

def generator_x():
    print("CALLING X GENERATOR!")
    return jnp.array([1., 2., 3., 4., 5.], dtype=np.float64)

layout = ak.contents.ListOffsetArray(
    ak.index.Index(VirtualArray(numpy, (4,), np.int64, generator_offsets)),
    ak.contents.NumpyArray(VirtualArray(jax, (5,), np.float64, generator_x)),
)
array = ak.materialize(layout)
val_mean, grad_mean = jaxlib.value_and_grad(ak.mean, argnums=0)(array)
_, grad_sum = jaxlib.value_and_grad(ak.sum, argnums=0)(array)
val_count, grad_count = jaxlib.value_and_grad(ak.count, argnums=0)(array)
assert val_mean == 3
assert ak.all(
    grad_mean == ak.Array([[0.2, 0.2, 0.2], [], [0.2, 0.2]], backend="jax")
)

# mean is treated as scaled sum
assert ak.all(grad_mean == grad_sum / val_count)

assert val_count == 5
assert ak.all(
    grad_count == ak.Array([[0.0, 0.0, 0.0], [], [0.0, 0.0]], backend="jax")
)

In this test, the ak.mean is the issue in the testing branch. The following works totally fine on this PR's branch but raises

ValueError: not enough values to unpack (expected 3, got 0)

This error occurred while calling

    ak.mean(
        <Array [[1.0, 2.0, 3.0], [], [4.0, 5.0]] type='3 * var * float64'>
    )

in the testing branch

import awkward as ak
ak.jax.register_and_check()
import numpy as np
import jax.numpy as jnp
import jax as jaxlib

from awkward._nplikes.virtual import VirtualArray, materialize_if_virtual
from awkward._nplikes.numpy import Numpy
from awkward._nplikes.jax import Jax
from awkward._nplikes.shape import unknown_length

numpy = Numpy.instance()
jax = Jax.instance()

def generator_offsets():
    print("CALLING OFFSETS GENERATOR!")
    return np.array([0, 3, 3, 5], dtype=np.int64)

def generator_x():
    print("CALLING X GENERATOR!")
    return jnp.array([1., 2., 3., 4., 5.], dtype=np.float64)

layout = ak.contents.ListOffsetArray(
    ak.index.Index(VirtualArray(numpy, (4,), np.int64, generator_offsets)),
    ak.contents.NumpyArray(VirtualArray(jax, (5,), np.float64, generator_x)),
)
ak.mean(layout)

[ikrommyd]

@ianna @pfackeldey let me know what you think of the way I'm adding this. I'll probably just add some less extensive testing for jax-virtual arrays.

[ikrommyd]

Needs #3457 and #3464