[FFI] Update load_inline interface

ffi/python/tvm_ffi/cpp/load_inline.py

@@ -326,10 +326,12 @@ def load_inline(
     cuda_sources: Sequence[str] | str, optional
         The CUDA source code. It can be a list of sources or a single source.
     functions: Mapping[str, str] | Sequence[str] | str, optional
-        The functions in cpp_sources that will be exported to the tvm ffi module. When a mapping is given, the keys
-        are the names of the exported functions, and the values are docstrings for the functions. When a sequence or a
-        single string is given, they are the functions needed to be exported, and the docstrings are set to empty
-        strings. A single function name can also be given as a string.
+        The functions in cpp_sources or cuda_source that will be exported to the tvm ffi module. When a mapping is
+        given, the keys are the names of the exported functions, and the values are docstrings for the functions. When
+        a sequence or a single string is given, they are the functions needed to be exported, and the docstrings are set
+        to empty strings. A single function name can also be given as a string. When cpp_sources is given, the functions
+        must be declared (not necessarily defined) in the cpp_sources. When cpp_sources is not given, the functions
+        must be defined in the cuda_sources. If not specified, no function will be exported.
     extra_cflags: Sequence[str], optional
         The extra compiler flags for C++ compilation.
         The default flags are:
@@ -369,6 +371,7 @@ def load_inline(
     elif isinstance(cuda_sources, str):
         cuda_sources = [cuda_sources]
     cuda_source = "\n".join(cuda_sources)
+    with_cpp = len(cpp_sources) > 0
     with_cuda = len(cuda_sources) > 0
 
     extra_ldflags = extra_ldflags or []
@@ -381,8 +384,13 @@ def load_inline(
         functions = {functions: ""}
     elif isinstance(functions, Sequence):
         functions = {name: "" for name in functions}
-    cpp_source = _decorate_with_tvm_ffi(cpp_source, functions)
-    cuda_source = _decorate_with_tvm_ffi(cuda_source, {})
+
+    if with_cpp:
+        cpp_source = _decorate_with_tvm_ffi(cpp_source, functions)
+        cuda_source = _decorate_with_tvm_ffi(cuda_source, {})
+    else:
+        cpp_source = _decorate_with_tvm_ffi(cpp_source, {})
+        cuda_source = _decorate_with_tvm_ffi(cuda_source, functions)
 
     # determine the cache dir for the built module
     if build_directory is None:

ffi/tests/python/test_load_inline.py

@@ -159,9 +159,6 @@ def test_load_inline_cpp_build_dir():
 def test_load_inline_cuda():
     mod: Module = tvm_ffi.cpp.load_inline(
         name="hello",
-        cpp_sources=r"""
-            void add_one_cuda(DLTensor* x, DLTensor* y);
-        """,
         cuda_sources=r"""
             __global__ void AddOneKernel(float* x, float* y, int n) {
               int idx = blockIdx.x * blockDim.x + threadIdx.x;