[compiler][python] Make target_backends optional (iree-org#18151)

When compiling through the iree-compile executable don't require the
target backend(s) to be specified.
It should be allowed to specify multiple target devices instead of
multiple backends.
This is inline with the deprecation of the target backends CLI option.

---------

Signed-off-by: Boian Petkantchin <boian.petkantchin@amd.com>

compiler/bindings/python/iree/compiler/tools/core.py

@@ -176,17 +176,16 @@ def build_compile_command_line(
       List of strings of command line.
     """
     iree_compile = find_tool("iree-compile")
-    if not options.target_backends:
-        raise ValueError("Expected a non-empty list for 'target_backends'")
 
     cl = [
         iree_compile,
         input_file,
         f"--iree-input-type={options.input_type!s}",
         f"--iree-vm-bytecode-module-output-format={options.output_format!s}",
     ]
-    for target_backend in options.target_backends:
-        cl.append(f"--iree-hal-target-backends={target_backend}")
+    if options.target_backends is not None:
+        for target_backend in options.target_backends:
+            cl.append(f"--iree-hal-target-backends={target_backend}")
 
     # Output file.
     if options.output_file:

compiler/bindings/python/test/tools/compiler_core_test.py

@@ -32,12 +32,6 @@ def testQueryTargets(self):
         # The VMVX target is always enabled.
         self.assertIn("vmvx", target_names)
 
-    def testNoTargetBackends(self):
-        with self.assertRaisesRegex(
-            ValueError, "Expected a non-empty list for 'target_backends'"
-        ):
-            binary = iree.compiler.tools.compile_str(SIMPLE_MUL_ASM)
-
     def testCompileStr(self):
         binary = iree.compiler.tools.compile_str(
             SIMPLE_MUL_ASM, target_backends=iree.compiler.tools.DEFAULT_TESTING_BACKENDS