initialize loss with first constraint loss instead of 0.0, and change random seed of euler_beam for better l2error

Author: HydrogenSulfate

examples/euler_beam/conf/euler_beam.yaml

@@ -23,7 +23,7 @@ hydra:
 
 # general settings
 mode: train # running mode: train/eval
-seed: 42
+seed: 1234
 output_dir: ${hydra:run.dir}
 log_freq: 20
 to_static: false

ppsci/loss/mtl/sum.py

@@ -43,8 +43,8 @@ def __call__(
         ), f"Number of given losses({len(losses)}) can not be empty."
         self.step = step
 
-        loss = 0.0
-        for i in range(len(losses)):
+        loss = losses[0]
+        for i in range(1, len(losses)):
             loss += losses[i]
 
         return loss