[ONNX] Fix ignored small eps in layer normalization in fp16 (#89869)
Prior to this change, the symbolic_fn `layer_norm` (before ONNX version 17) always lose precision when eps is smaller than Float type, while PyTorch always take eps as Double. This PR adds `onnx::Cast` into eps related operations to prevent losing precision during the calculation.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/89869
Approved by: https://github.com/BowenBao
