fix cpp builder to avoid missing-source compile error (pytorch#160354)

Summary:
the condition
```
if config.is_fbcode() and (not self._aot_mode or self._use_relative_path):
    sources = [os.path.basename(i) for i in sources]
```
unintentionally (?) stripped paths even when use_relative_path was False (as long as aot_mode was False), breaking local tests that rely on absolute temp-file paths.

Fixes internal issue:
```

FAILED (errors=1)

CppCompileError: C++ compile error

Command:
/mnt/gvfs/third-party2/llvm-fb/0f1f083aa5508772f3db24bf4f697bc118ba0958/17/platform010/72a2ff8/bin/clang-17 czyi3nhzin5b3mc3376vmfnlbjobvjcghbvv4tatuazs3syqubay.cpp -shared -fPIC -O3 -DNDEBUG -fno-trapping-math -funsafe-math-optimizations -ffinite-math-only -fno-signed-zeros -fno-math-errno -fno-finite-math-only -fno-unsafe-math-optimizations -ffp-contract=off -Wall -std=c++17 -Wno-unused-variable -Wno-unknown-pragmas -Werror=ignored-optimization-argument -g -o /re_tmp/tmpsp58ya2h/zy/test_symbol.so

Output:
clang-17: error: no such file or directory: 'czyi3nhzin5b3mc3376vmfnlbjobvjcghbvv4tatuazs3syqubay.cpp'
clang-17: error: no input files
```

Reviewed By: clee2000

Differential Revision: D80025417

Pull Request resolved: pytorch#160354
Approved by: https://github.com/benjaminglass1, https://github.com/clee2000

Author: izaitsevfb

torch/_inductor/cpp_builder.py

@@ -1631,7 +1631,8 @@ def __init__(
         if isinstance(sources, str):
             sources = [sources]
 
-        if config.is_fbcode() and (not self._aot_mode or self._use_relative_path):
+        # Use relative paths only when requested (typically for remote builds)
+        if config.is_fbcode() and self._use_relative_path:
             # Will create another temp directory for building, so do NOT use the
             # absolute path.
             self._orig_source_paths = list(sources)