[Relax][PyTorch] Fix AssertionError: Unsupported function types `randn.default`

[cchung100m]

This PR is trying to fix issues https://github.com/apache/tvm/issues/18362.

For now, return zeros with the correct shape since random initialization randn is mainly used during training, not inference. This is a temporary solution until Relax adds proper random number generation support.