Simplify MuJoCo Studio viewer selection and unify model flags.

This change removes the ViewerMode enum and instead infers whether to launch the Web Viewer or Native Viewer based on the graphics mode (gfx) setting. Specifically, setting gfx to "web" or "webgl" will now launch the Web Viewer (in future "webgpu" would also launch the Web Viewer).

Additionally, this unifies command-line flags across studio scripts and samples by replacing the --mjcf flag with --model (with positional argument fallback) and standardizing absl flags usage. --model is a better name since formats like .mjz and .mjb are also supported by this argument.

PiperOrigin-RevId: 959058940
Change-Id: If4e0ace664ddc6e45cb681d168679148e8c4901d

python/mujoco/experimental/studio/sample/ghost.py

@@ -38,12 +38,15 @@ from mujoco.experimental.dear_imgui import dear_imgui as imgui
 
 vp = viewer_protocol
 
-_GFX = _flags.DEFINE_enum('gfx', None, vp.GFX_MODES, 'Graphics mode.')
+_MODEL = _flags.DEFINE_string('model', None, 'Path to model file.')
+_GFX = _flags.DEFINE_enum(
+    'gfx', None, vp.GFX_MODES, 'Graphics mode ("web" launches Web Viewer).'
+)
+_PORT = _flags.DEFINE_integer(
+    'port', 0, 'Web Viewer port (0 picks first free port >= 8080).'
+)
 _WIDTH = _flags.DEFINE_integer('width', 1200, 'Width of the output image.')
 _HEIGHT = _flags.DEFINE_integer('height', 800, 'Height of the output image')
-_VIEWER = _flags.DEFINE_enum_class(
-    'viewer', vp.ViewerMode.NATIVE, vp.ViewerMode, 'Viewer mode.'
-)
 
 
 class GhostRenderer:
@@ -155,15 +158,18 @@ class GhostRenderer:
 
 
 def main(argv: list[str]) -> None:
-  if len(argv) != 2:
-    raise _app.UsageError('Please provide exactly one MJCF path argument.')
+  model_path = _MODEL.value or (
+      argv[1] if len(argv) > 1 and not argv[1].startswith('--') else None
+  )
+  if not model_path:
+    raise _app.UsageError('Please provide a model path argument or --model flag.')
 
   data = None
   try:
-    if (data := parser.parse(argv[1])) is None:
+    if (data := parser.parse(model_path)) is None:
       raise ValueError('parser returned None')
   except Exception as ex:  # pylint: disable=broad-except
-    print(f'Failed to load model from {argv[1]!r}: {ex}')
+    print(f'Failed to load model from {model_path!r}: {ex}')
     sys.exit(1)
   model = data.model
 
@@ -172,7 +178,7 @@ def main(argv: list[str]) -> None:
       width=_WIDTH.value,
       height=_HEIGHT.value,
       gfx=_GFX.value or '',
-      viewer_mode=_VIEWER.value,
+      http_port=_PORT.value,
   )
 
   ghost_renderer = GhostRenderer()