Address issue with AWS instance type schema (#2787)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

src/_nebari/stages/infrastructure/__init__.py

@@ -128,7 +128,7 @@ class AzureInputVars(schema.Base):
     workload_identity_enabled: bool = False
 
 
-class AWSAmiTypes(enum.Enum):
+class AWSAmiTypes(str, enum.Enum):
     AL2_x86_64 = "AL2_x86_64"
     AL2_x86_64_GPU = "AL2_x86_64_GPU"
     CUSTOM = "CUSTOM"
@@ -151,25 +151,17 @@ class AWSNodeGroupInputVars(schema.Base):
     ami_type: Optional[AWSAmiTypes] = None
     launch_template: Optional[AWSNodeLaunchTemplate] = None
 
-    @field_validator("ami_type", mode="before")
-    @classmethod
-    def _infer_and_validate_ami_type(cls, value, values) -> str:
-        gpu_enabled = values.get("gpu", False)
-
-        # Auto-set ami_type if not provided
-        if not value:
-            if values.get("launch_template") and values["launch_template"].ami_id:
-                return "CUSTOM"
-            if gpu_enabled:
-                return "AL2_x86_64_GPU"
-            return "AL2_x86_64"
-
-        # Explicit validation
-        if value == "AL2_x86_64" and gpu_enabled:
-            raise ValueError(
-                "ami_type 'AL2_x86_64' cannot be used with GPU enabled (gpu=True)."
-            )
-        return value
+
+def construct_aws_ami_type(gpu_enabled: bool, launch_template: AWSNodeLaunchTemplate):
+    """Construct the AWS AMI type based on the provided parameters."""
+
+    if launch_template and launch_template.ami_id:
+        return "CUSTOM"
+
+    if gpu_enabled:
+        return "AL2_x86_64_GPU"
+
+    return "AL2_x86_64"
 
 
 class AWSInputVars(schema.Base):
@@ -858,6 +850,10 @@ def input_vars(self, stage_outputs: Dict[str, Dict[str, Any]]):
                         single_subnet=node_group.single_subnet,
                         permissions_boundary=node_group.permissions_boundary,
                         launch_template=node_group.launch_template,
+                        ami_type=construct_aws_ami_type(
+                            gpu_enabled=node_group.gpu,
+                            launch_template=node_group.launch_template,
+                        ),
                     )
                     for name, node_group in self.config.amazon_web_services.node_groups.items()
                 ],