[Bug Fix] Enabling custom op name without adding stack frame locations (#9676)

This PR fixes two related issues in `PopulateXlaOpMetadata` that
prevented proper usage of `CustomOpNameMetaData`:
## Fix 1: Support custom op names without stack frame locations 

This will enable user to change the name of the op using
`CustomOpNameMetaData` without necessarily having to add stack frame
locations. Example of usage (if we pass 0 for `max_stack_depth` field it
will just prepend custom prefix to the current op name):
```
torch_xla._XLAC._set_xla_custom_op_name_prefix(tensor, your_custom_name_prefix, 0)
```
Additionaly, this will guard the `AddStackFrameLocations()` function
from passing invalid number for the `max_stack_depth`. If we were to
pass number that is <= 0, we would get a **segmentation fault** due to
improper iterator dereferencing (which could've happen before this
change).

The `AddStackFrameLocations()` function in
`stack_frame_index_builder.cpp` uses reverse iterators and assumes at
least one iteration occurs:
```cpp
  auto frame_it = frame_info.rbegin();
  for (; frame_it != frame_info.rend() && depth < max_stack_depth; ++frame_it) {
    // Loop never executes when max_stack_depth == 0
  }
  --frame_it;  // ← Segfault: iterator is still at rbegin(), decrement goes past-the-end
  metadata_to_populate.set_source_file(frame_it->file);  // ← Dereference invalid iterator
  ```
  
## Fix 2: Prevent scope from overwriting custom metadata

**Problem:** 

Even when users set custom metadata via `_set_xla_custom_op_name_prefix()`, the `nmeta.scope` field was unconditionally overwriting the custom `op_name_prefix`. This affected operations like `add` and `mul` which have scope set (e.g., `aten::add.3`), resulting in loss of user-provided semantic location information.                                                                                                                                                                                                                                    

**Changes:**                                                                                                                                                                                                                                                                            

- Modified the condition from `if (!nmeta.scope.empty())` to `else if (!nmeta.scope.empty())`                                                                                                                                                                                           
- This ensures custom metadata takes precedence: custom metadata is used if available, otherwise scope is used as fallback

                                                                                                                                                             
**Precedence hierarchy** (now correctly implemented):                                                                                                                                                                                                                                   

1. Custom user metadata (via `SetUserMetadata` APIs) - highest priority                                                                                                                                                                                                                 
2. Scope-based naming (auto-generated by torch-xla) - fallback                                                                                                                                                                                                                          
3. Bare op_type - default

Author: vkovinicTT

torch_xla/csrc/lowering_context.cpp

@@ -78,15 +78,19 @@ class HloMetadataSetter {
       max_stack_depth = custom_opname_meta->max_stack_depth;
     }
 
-    if (!nmeta.scope.empty()) {
+    else if (!nmeta.scope.empty()) {
       op_name_prefix =
           absl::StrCat(absl::StrReplaceAll(nmeta.scope, {{":", "_"}}), "/");
     }
     metadata.set_op_name(absl::StrCat(op_name_prefix, op_type));
 
-    // Sets file, line and stack_frame_id in metadata
-    lowering_context.stack_frame_index_builder()->AddStackFrameLocations(
-        nmeta.frame_info, static_cast<int>(max_stack_depth), metadata);
+    // NOTE: if max_stack_depth is 0, we are just renaming the op, so we don't
+    // need to add stack frame locations
+    if (max_stack_depth > 0) {
+      // Sets file, line and stack_frame_id in metadata
+      lowering_context.stack_frame_index_builder()->AddStackFrameLocations(
+          nmeta.frame_info, static_cast<int>(max_stack_depth), metadata);
+    }
 
     lowering_context.builder()->SetOpMetadata(std::move(metadata));
   }