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/viewer.py

@@ -14,7 +14,7 @@
 """Interactive Studio GUI viewer for MuJoCo."""
 
 from absl import app as _app
-from absl import flags
+from absl import flags as _flags
 from mujoco.experimental.studio import launch_passive
 from mujoco.experimental.studio import messages
 from mujoco.experimental.studio import parser
@@ -24,19 +24,15 @@ from mujoco.experimental.studio import viewer_protocol
 
 vp = viewer_protocol
 
-_GFX = flags.DEFINE_enum('gfx', None, vp.GFX_MODES, 'Graphics mode.')
-_WIDTH = flags.DEFINE_integer('width', 1200, 'Width of output window.')
-_HEIGHT = flags.DEFINE_integer('height', 800, 'Height of output window.')
-_MJCF_PATH = flags.DEFINE_string('mjcf', None, 'Path to MJCF file.')
-_VIEWER = flags.DEFINE_enum_class(
-    'viewer', vp.ViewerMode.NATIVE, vp.ViewerMode, 'Viewer 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 the first free port starting at 8080, so '
-    'several viewers can run side by side.',
+_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')
 
 
 def main(argv: list[str]) -> None:
@@ -44,12 +40,11 @@ def main(argv: list[str]) -> None:
       width=_WIDTH.value,
       height=_HEIGHT.value,
       gfx=_GFX.value or '',
-      viewer_mode=_VIEWER.value,
       http_port=_PORT.value,
   )
 
   # Resolve model path, if provided.
-  model_path = _MJCF_PATH.value or (
+  model_path = _MODEL.value or (
       argv[1] if len(argv) > 1 and not argv[1].startswith('--') else None
   )