Add `reward` and `observation` arguments to env.reset()

🚀 Feature

The CompilerEnv constructor accepts a pair of arguments reward_space and observation_space. We should add those to env.reset(), same as specifying the benchmark.

Motivation

Because this feels like a clumsy API:

env.reward_space = "Foobar"
env.reset(benchmark="benchmark://foo-v0/abc")

Pitch

Allow:

env.reset(benchmark="benchmark://foo-v0/abc", reward_space="Foobar")

[uduse]

env.reset(
    benchmark=compiler_gym.ValueNotChanged,
    reward_space="some_new_space",
    observation_space=None,  # no observations
)

This seems good to me. Alternatively, we could use python’s ellipsis ... as this special placeholder, or just the string "same". Personally I prefer just "same" because it’s simple, explicit, easy to understand, and immutable which makes it a good choice as the default value.

[ChrisCummins]

I’m also not sure I like the default being the string "same", as it doesn’t feel particularly self-documenting, and prevents the (unlikely) use of the name “same” with observation/reward spaces.

I would suggest adding a new enum to compiler_gym/util/gym_type_hints.py

from enum import Enum

class OptionalArgumentValue(Enum):
    UNCHANGED = 1

The OptionalArgumentValue.UNCHANGED could be used as the default value and checked for using:

if reward_space != OptionalArgumentValue.UNCHANGED:
    ...

Cheers, Chris