feat: add `PyStructSequence` types as internal node types

[XuehaiPan]

Description

Describe your changes in detail.

Add PyStructSequence types as internal node types.

Motivation and Context

Why is this change required? What problem does it solve? If it fixes an open issue, please link to the issue here. You can use the syntax close #15213 if this solves the issue #15213

• [X] I have raised an issue to propose this change (required for new features and bug fixes)

Resolves #29.

Without this PR: StructSequences are leaves

>>> import sys
>>> import torch
>>> from optree import *

>>> tree_flatten(sys.float_info)
(
    [sys.float_info(max=1.7976931348623157e+308, max_exp=1024, max_10_exp=308, min=2.2250738585072014e-308, min_exp=-1021, min_10_exp=-307, dig=15, mant_dig=53, epsilon=2.220446049250313e-16, radix=2, rounds=1)],
    PyTreeSpec(*)
)

>>> tree_flatten(sys.int_info)
(
    [sys.int_info(bits_per_digit=30, sizeof_digit=4, default_max_str_digits=4300, str_digits_check_threshold=640)],
    PyTreeSpec(*)
)

>>> tree_flatten(torch.max(torch.arange(12).reshape(3, 4), dim=-1))
(
    [torch.return_types.max(values=tensor([ 3,  7, 11]),
                            indices=tensor([3, 3, 3]))],
    PyTreeSpec(*)
)

With this PR: StructSequences behave like tuples

>>> tree_flatten(sys.float_info)
(
    [1.7976931348623157e+308, 1024, 308, 2.2250738585072014e-308, -1021, -307, 15, 53, 2.220446049250313e-16, 2, 1],
    PyTreeSpec(sys.float_info(max=*, max_exp=*, max_10_exp=*, min=*, min_exp=*, min_10_exp=*, dig=*, mant_dig=*, epsilon=*, radix=*, rounds=*))
)

>>> tree_flatten(sys.int_info)
(
    [30, 4, 4300, 640],
    PyTreeSpec(sys.int_info(bits_per_digit=*, sizeof_digit=*, default_max_str_digits=*, str_digits_check_threshold=*))
)

>>> tree_flatten(torch.max(torch.arange(12).reshape(3, 4), dim=-1))
(
    [tensor([ 3,  7, 11]), tensor([3, 3, 3])],
    PyTreeSpec(torch.return_types.max(values=*, indices=*))
)

New helper functions:

• is_structseq
• is_structseq_class
• structseq_fields

>>> is_structseq(sys.float_info)
True
>>> is_structseq(type(sys.float_info))
True
>>> is_structseq_class(sys.float_info)
False
>>> is_structseq_class(type(sys.float_info))
True
>>> structseq_fields(sys.float_info)
('max', 'max_exp', 'max_10_exp', 'min', 'min_exp', 'min_10_exp', 'dig', 'mant_dig', 'epsilon', 'radix', 'rounds')
>>> structseq_fields(type(sys.float_info))
('max', 'max_exp', 'max_10_exp', 'min', 'min_exp', 'min_10_exp', 'dig', 'mant_dig', 'epsilon', 'radix', 'rounds')

Types of changes

What types of changes does your code introduce? Put an x in all the boxes that apply:

• [ ] Bug fix (non-breaking change which fixes an issue)
• [X] New feature (non-breaking change which adds core functionality)
• [X] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Documentation (update in the documentation)

Checklist

Go over all the following points, and put an x in all the boxes that apply. If you are unsure about any of these, don't hesitate to ask. We are here to help!

• [X] I have read the CONTRIBUTION guide. (required)
• [X] My change requires a change to the documentation.
• [X] I have updated the tests accordingly. (required for a bug fix or a new feature)
• [X] I have updated the documentation accordingly.
• [X] I have reformatted the code using make format. (required)
• [X] I have checked the code using make lint. (required)
• [X] I have ensured make test pass. (required)

[codecov[bot]]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage :thumbsup:

Coverage data is based on head (0b6aa78) compared to base (b41db31). Patch coverage: 100.00% of modified lines in pull request are covered.

:exclamation: Current head 0b6aa78 differs from pull request most recent head 25d3872. Consider uploading reports for the commit 25d3872 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #30 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 4 4 Lines 347 351 +4 ========================================= + Hits 347 351 +4 ``` | Flag | Coverage Δ | | |---|---|---| | unittests | `100.00% <100.00%> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=metaopt#carryforward-flags-in-the-pull-request-comment) to find out more. | [Impacted Files](https://codecov.io/gh/metaopt/optree/pull/30?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=metaopt) | Coverage Δ | | |---|---|---| | [optree/\_\_init\_\_.py](https://codecov.io/gh/metaopt/optree/pull/30?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=metaopt#diff-b3B0cmVlL19faW5pdF9fLnB5) | `100.00% <ø> (ø)` | | | [optree/ops.py](https://codecov.io/gh/metaopt/optree/pull/30?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=metaopt#diff-b3B0cmVlL29wcy5weQ==) | `100.00% <100.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=metaopt). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=metaopt)

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