Hi TN date class accuracy improvement

[shrpawar-alt]

What does this PR do ?

Improved Date class accuracy from ~87 % to ~99 % by introducing additional graph coverage for the cases failing earlier.

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PR Type:

• New Feature
• Bugfix
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[mgrafu]

can l36 through here be replaced by string file?

[shrpawar-alt]

Thanks for the suggestion! I explored pynini.string_file() for both. It works for suffix_union if we add identity columns to suffixes.tsv, since suffix comes after graph_year and doesn't affect year graph path selection. However, for prefix_union, even with identity columns, string_file() caused graph_year_thousands to incorrectly win over graph_year_hundreds_as_thousands for years like 1999, 1920, 1971, because prefix_union comes before graph_year in the concatenation, and the identity transducer from string_file() alters the weight landscape at that point.
I considered adjusting weights to compensate, but since graph_year_thousands and graph_year_hundreds_as_thousands are designed to be mutually exclusive, it was unclear why string_file() was breaking that exclusivity, making weight tuning risky. To keep both consistent and avoid that risk, I've now used pynini.string_map() for both, as it eliminates the intermediate list variables and the verbose open() + pynini.union() block while correctly handling single-column entries and preserving the expected FST behavior.

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