[Chaitanya140904]

This PR adds support for the ONNX DFT operator in the Relax ONNX frontend, resolving #20960.

Key Changes

• In python/tvm/relax/frontend/onnx/onnx_frontend.py:
  • Implemented DFT op converter supporting opset 17 and 20.
  • Supports forward and inverse DFT, full spectrum and one-sided spectrum (onesided=1).
  • Supports both real ([..., 1]) and complex ([..., 2]) inputs.
  • Supports axis as attribute (opset 17) and axis as input tensor (opset 20).
  • Handles signal zero-padding and truncation with dft_length.
  • Formulates transform via matrix multiplication with precomputed trigonometric matrices (C = cos(2*pi*n*k/N), S = sin(2*pi*n*k/N)) over Relax ops.
  • Registered "DFT": DFT in _get_convert_map().
• In tests/python/relax/test_frontend_onnx.py:
  • Added test_dft covering real forward full, real forward onesided, complex inverse, dft_length padding/truncating, axis=0, and opset 17/20, validated against ONNX Runtime.

Closes #20960.

[Chaitanya140904]

Hi @mshr-h, @yzhliu, @tqchen, @akaashrp

Could you please take a look at this PR when you get a chance?