Compute validation metrics at first step (#1508)

Currently, the first time validation metrics are computed is when `step
== job_config.validation.freq`. I think it is preferable to always
compute them for the first step as well.

Author: runame

torchtitan/components/validate.py

@@ -28,7 +28,7 @@ def validate(self, model_parts: list[nn.Module]) -> dict[str, float]:
         raise NotImplementedError("validate method not implemented")
 
     def should_validate(self, step: int) -> bool:
-        return step % self.job_config.validation.freq == 0
+        return step == 1 or step % self.job_config.validation.freq == 0
 
 
 class Validator(BaseValidator):