[Kernel] Update tile tuning key for future mixed precision support

[kyuyeunk]

Description

Update tile tuning key for future mixed precision support. Note that this PR does not introduce any new features.

• add argument x_q_dtype that specifies the precision of activation quantization
• add extra field to kernel tile tuning key specifying activation quantization dtype
• make tile tuning key/value data structure flexible by introducing classes TunedKey and TunedValue
• general formatting & readability improvements

Tests

pytest -v tests/kernels/quantized_matmul_kernel_test.py

https://buildkite.com/tpu-commons/tpu-commons-ci/builds/3156

Checklist

Before submitting this PR, please make sure:

• I have performed a self-review of my code.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have made or will make corresponding changes to any relevant documentation.

[github-actions]

Description

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Tests

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Checklist

Before submitting this PR, please make sure:

• I have performed a self-review of my code.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have made or will make corresponding changes to any relevant documentation.