DeepXDE v1.5.0

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