Update throughput docs (#19415)

Author: carmocca

docs/source-fabric/index.rst

@@ -211,6 +211,7 @@ Get Started
     Loggers <api/loggers>
     Precision <api/precision>
     Strategies <api/strategies>
+    Utilities <api/utilities>
 
 
 .. toctree::

src/lightning/pytorch/callbacks/throughput_monitor.py

@@ -50,7 +50,7 @@ def setup(self, stage):
                     model = MyModel()
 
                     def sample_forward():
-                        batch = torch.randn(...)
+                        batch = torch.randn(..., device="meta")
                         return model(batch)
 
                     self.flops_per_batch = measure_flops(model, sample_forward, loss_fn=torch.Tensor.sum)