Transducer Decoding: avoid alignments

[artbataev]

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What does this PR do ?

Store non-blank confidence in BatchedHyps to avoid costly alignments structure for confidence when possible (if blank label confidence is ignored):

• confidence is used in different applications, but current implementation with alignments makes decoding significantly slower
• by default blank label confidence is ignored
• WIND RNN-T decoding (making RNN-T as fast as TDT) is incompatible with saving results at each step. This PR unblocks Faster RNN-T Label-Looping Decoding with WIND #14451

Additionally, this PR unifies and simplifies RNN-T and TDT decoding.

Collection: [ASR]

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