[Relax][Frontend][TFLite] Add bucketize/unique/sign + fixes

[0xjah]

Summary

• add Relax TFLite handlers for BUCKETIZE, UNIQUE, SIGN, and RELU_0_TO_1
• allow RANGE with dynamic scalar operands and guard FAKE_QUANT degenerate ranges
• extend TFLite frontend tests for sign/unique/bucketize/fake_quant and dynamic range

Testing

• not run (not requested)

[gemini-code-assist]

Code Review

This pull request adds support for several TFLite operators—BUCKETIZE, RELU_0_TO_1, SIGN, and UNIQUE—to the Relax frontend and enables dynamic scalar inputs for the RANGE operator. It also hardens FAKE_QUANT against division-by-zero errors in degenerate ranges. Review feedback identifies a semantic mismatch in the BUCKETIZE implementation regarding boundary matching, suggests using math.floor(x + 0.5) for TFLite-compliant rounding in quantized RELU_0_TO_1, and recommends adding a specific test case for the FAKE_QUANT degenerate range fix.