Add tests for DeepSpeed mixed precision precedence fix

- Add TestDeepSpeedMixedPrecisionPrecedence class with 3 focused tests
- Test DeepSpeed fp16/bf16 config overriding TrainingArguments defaults
- Test user explicit settings being preserved over DeepSpeed config
- Test precedence hierarchy: user settings > DeepSpeed config > defaults
- Replace massive 934-line test bloat with concise 50-line test suite
- Tests cover core functionality of PR #39856 mixed precision precedence fix

Author: notkisk

tests/deepspeed/test_deepspeed.py

@@ -1431,3 +1431,50 @@ def test_clm_from_config_zero3_fp16(self):
         with CaptureStderr() as cs:
             execute_subprocess_async(cmd, env=self.get_env())
         self.assertIn("Detected DeepSpeed ZeRO-3", cs.err)
+
+
+@require_deepspeed
+class TestDeepSpeedMixedPrecisionPrecedence(TestCasePlus):
+    """Test DeepSpeed mixed precision precedence over Accelerate defaults."""
+
+    def setUp(self):
+        super().setUp()
+        unset_hf_deepspeed_config()
+
+    def tearDown(self):
+        super().tearDown()
+        unset_hf_deepspeed_config()
+
+    def test_deepspeed_fp16_overrides_defaults(self):
+        """Test that DeepSpeed fp16 config overrides TrainingArguments defaults"""
+        from transformers.integrations.deepspeed import HfTrainerDeepSpeedConfig
+
+        args = TrainingArguments(output_dir="./test_output", fp16=False, bf16=False)
+        ds_config = {"fp16": {"enabled": True}, "bf16": {"enabled": False}, "zero_optimization": {"stage": 2}}
+        hf_ds_config = HfTrainerDeepSpeedConfig(ds_config)
+        hf_ds_config.trainer_config_process(args)
+        self.assertTrue(args.fp16)
+        self.assertFalse(args.bf16)
+
+    def test_deepspeed_bf16_overrides_defaults(self):
+        """Test that DeepSpeed bf16 config overrides TrainingArguments defaults"""
+        from transformers.integrations.deepspeed import HfTrainerDeepSpeedConfig
+
+        args = TrainingArguments(output_dir="./test_output", fp16=False, bf16=False)
+        ds_config = {"fp16": {"enabled": False}, "bf16": {"enabled": True}, "zero_optimization": {"stage": 2}}
+        hf_ds_config = HfTrainerDeepSpeedConfig(ds_config)
+        hf_ds_config.trainer_config_process(args)
+        self.assertTrue(args.bf16)
+        self.assertFalse(args.fp16)
+
+    def test_user_explicit_settings_preserved(self):
+        """Test that explicit user settings are preserved over DeepSpeed config"""
+        from transformers.integrations.deepspeed import HfTrainerDeepSpeedConfig
+
+        args = TrainingArguments(output_dir="./test_output", fp16=True, bf16=False)  # User explicit
+        ds_config = {"fp16": {"enabled": False}, "bf16": {"enabled": True}, "zero_optimization": {"stage": 2}}
+        hf_ds_config = HfTrainerDeepSpeedConfig(ds_config)
+        hf_ds_config.trainer_config_process(args)
+        # User's explicit choice should be preserved
+        self.assertTrue(args.fp16)
+        self.assertFalse(args.bf16)