Incorrect setting for num_beams in translation and summarization examples (#27519)

* Remove the torch main_process_first context manager from TF examples

* Correctly set num_beams=1 in our examples, and add a guard in GenerationConfig.validate()

* Update src/transformers/generation/configuration_utils.py

Co-authored-by: amyeroberts <22614925+amyeroberts@users.noreply.github.com>

---------

Co-authored-by: amyeroberts <22614925+amyeroberts@users.noreply.github.com>

examples/pytorch/summarization/run_summarization.py

@@ -249,7 +249,7 @@ class DataTrainingArguments:
         },
     )
     num_beams: Optional[int] = field(
-        default=None,
+        default=1,
         metadata={
             "help": (
                 "Number of beams to use for evaluation. This argument will be passed to ``model.generate``, "