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/implot.py

@@ -17,7 +17,7 @@ This script runs a Studio viewer and adds an 'Inspect Body' window using ImGui
 and ImPlot bindings to visualize selected body data. The example demonstrates
 how responsive UI layout rules are easily implemented.
 
-Provide an MJCF model file via the first command-line argument to launch.
+Provide a model file via the --model flag or positional argument to launch.
 """
 
 import math
@@ -40,12 +40,15 @@ from mujoco.experimental.implot import implot
 
 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.'
-)
 
 
 _N_HISTORY = 100
@@ -211,12 +214,15 @@ class BodyInspector:
 
 
 def main(argv: list[str]) -> None:
-  if len(argv) < 2:
-    print('Usage: implot <model_path.xml>')
+  model_path = _MODEL.value or (
+      argv[1] if len(argv) > 1 and not argv[1].startswith('--') else None
+  )
+  if not model_path:
+    print('Usage: implot --model=<model_path.xml>')
     sys.exit(1)
 
-  if (data := parser.parse(argv[1])) is None:
-    print(f'Error loading model from {argv[1]!r}')
+  if (data := parser.parse(model_path)) is None:
+    print(f'Error loading model from {model_path!r}')
     sys.exit(1)
   model = data.model
 
@@ -227,7 +233,7 @@ def main(argv: list[str]) -> None:
       width=_WIDTH.value,
       height=_HEIGHT.value,
       gfx=_GFX.value or '',
-      viewer_mode=_VIEWER.value,
+      http_port=_PORT.value,
   )
 
   with launch_passive.launch_passive(