Smarter matching for default subcommands

[brentyi]

From nerfstudio-project/nerfstudio#1281:

When we pass in a default for a type with subcommands, tyro needs to figure out which subcommand the default corresponds to (is this config yaml from ns-train nerfacto? ns-train depth-nerfacto? ns-train instant-ngp?). For the vast majority of cases this matching is straightforward, but nerfstudio's subcommands happen to not fall into this category.

The goal of this PR is to improve matching for default subcommands.

In order of priority, we now match against:

• The hash of subcommand defaults. (via object.__hash__)
• The value of subcommand defaults. (via equality comparison)
• New: a notion of a type tree, of the specified default against subcommands with concrete defaults.
• New: a type tree, of the specified default against the subcommand annotations.

Needs tests!

[codecov]

Codecov Report

Base: 99.21% // Head: 99.21% // Decreases project coverage by -0.01% ⚠️

Coverage data is based on head (27a7f0b) compared to base (59f64bc).
Patch coverage: 95.38% of modified lines in pull request are covered.

❗ Current head 27a7f0b differs from pull request most recent head 7644779. Consider uploading reports for the commit 7644779 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #33      +/-   ##
==========================================
- Coverage   99.21%   99.21%   -0.01%     
==========================================
  Files          21       22       +1     
  Lines        1654     1648       -6     
==========================================
- Hits         1641     1635       -6     
  Misses         13       13              

Flag	Coverage Δ
unittests	99.21% <95.38%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
tyro/_subcommand_matching.py	94.82% <94.82%> (ø)
tyro/_fields.py	98.89% <100.00%> (+0.99%)	⬆️
tyro/_parsers.py	100.00% <100.00%> (ø)
tyro/_instantiators.py	99.01% <0.00%> (-0.05%)	⬇️
tyro/_cli.py	100.00% <0.00%> (ø)
tyro/_argparse_formatter.py	97.36% <0.00%> (ø)

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