(experimental) Propagate helptext into subcommands

[brentyi]

discussed in #221

from dataclasses import dataclass
import tyro

@dataclass(frozen=True)
class AdamConfig:
    adam_foo: float = 1.0

@dataclass(frozen=True)
class SGDConfig:
    sgd_foo: float = 1.0

@dataclass(frozen=True)
class Config:
    optimizer: AdamConfig | SGDConfig = AdamConfig()
    """Optimizer configuration."""

tyro.cli(Config)

root helptext (unchanged):

usage: ex.py [-h] [{optimizer:adam-config,optimizer:sgd-config}]

╭─ options ─────────────────────────────────────────────────╮
│ -h, --help        show this help message and exit         │
╰───────────────────────────────────────────────────────────╯
╭─ optional subcommands ────────────────────────────────────╮
│ Optimizer configuration. (default: optimizer:adam-config) │
│ ───────────────────────────────────────────────────────── │
│ [{optimizer:adam-config,optimizer:sgd-config}]            │
│     optimizer:adam-config                                 │
│     optimizer:sgd-config                                  │
╰───────────────────────────────────────────────────────────╯

subcommand helptext ("Optimizer configuration" is new):

usage: ex.py optimizer:adam-config [-h] [--optimizer.adam-foo FLOAT]

╭─ options ─────────────────────────────────────────╮
│ -h, --help        show this help message and exit │
╰───────────────────────────────────────────────────╯
╭─ optimizer options ───────────────────────────────╮
│ Optimizer configuration.                          │
│ ────────────────────────────────                  │
│ --optimizer.adam-foo FLOAT                        │
│                   (default: 1.0)                  │
╰───────────────────────────────────────────────────╯

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.82%. Comparing base (0c2cb26) to head (240be27).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #227   +/-   ##
=======================================
  Coverage   99.82%   99.82%           
=======================================
  Files          28       28           
  Lines        2304     2304           
=======================================
  Hits         2300     2300           
  Misses          4        4           

Flag	Coverage Δ
unittests	99.82% <ø> (ø)

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