ENH mini bench

[tomMoral]

Fixes #605.

This implements decorator to create mini benchmarks runnable with benchopt run --mini mini_file.py.
This allows to run the following benchmark:

from benchopt.mini import solver, dataset, objective
import jax

@dataset(
    size=100,
    random_state=0
)
def simulated(size, random_state):
    key = jax.random.PRNGKey(random_state)
    key, subkey = jax.random.split(key)
    X = jax.random.normal(key, (size,))
    return dict(X=X)


@solver(
    name="Solver 1",
    lr=[1e-2, 1e-3]
)
def solver1(n_iter, X, lr):
    beta = X
    for i in range(n_iter):
        beta -= lr * beta

    return dict(beta=beta)


@objective(name="Benchmark HVP")
def evaluate(beta):
    return dict(value=(0.5 * beta.dot(beta)).item())

Not completely sure this is useful but got it done by playing with another benchmark, it would be nice to have feedback on whether it seems usable or not.

(No doc or test yet)

Checks before merging PR

• added documentation for any new feature
• added unit test
• edited the what's new (if applicable)

[codecov-commenter]

Codecov Report

Merging #641 (724f685) into main (9955d4c) will decrease coverage by 1.21%.
The diff coverage is 29.62%.

❗ Current head 724f685 differs from pull request most recent head 51c1de3. Consider uploading reports for the commit 51c1de3 to get more accurate results

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