Areas of improvement
- Support new loss "mean l2 relative error"
- Backend PyTorch supports `dde.nn.DeepONetCartesianProd` and `dde.nn.PODDeepONet`
- Backend PyTorch supports "step" LR decay
- `dde.callbacks.OperatorPredictor` supports backends tensorflow.compat.v1, tensorflow, pytorch, paddle
- Fix `dde.config.set_random_seed`
New APIs
- Add `dde.config.enable_xla_jit` and `dde.config.disable_xla_jit`
API changes
- Change `DDEBACKEND` to `DDE_BACKEND`