[testing] fix ambiguous test (#6898)

Since `generate()` does:
```
        num_beams = num_beams if num_beams is not None else self.config.num_beams
```
This test fails if `model.config.num_beams > 1` (which is the case in the model I'm porting).

This fix makes the test setup unambiguous by passing an explicit `num_beams=1` to `generate()`.

Thanks.

tests/test_modeling_common.py

@@ -822,7 +822,7 @@ class ModelTesterMixin:
             with self.assertRaises(AssertionError):
                 # generating multiple sequences when no beam search generation
                 # is not allowed as it would always generate the same sequences
-                model.generate(input_ids, do_sample=False, num_return_sequences=2)
+                model.generate(input_ids, do_sample=False, num_beams=1, num_return_sequences=2)
 
             # num_return_sequences > 1, sample
             self._check_generated_ids(model.generate(input_ids, do_sample=True, num_return_sequences=2))