docs: update fastsafetensors usage instructions (#22891)

Signed-off-by: Nir Levy <bhr166@gmail.com>

docs/models/extensions/fastsafetensor.md

@@ -2,4 +2,5 @@ Loading Model weights with fastsafetensors
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 Using fastsafetensors library enables loading model weights to GPU memory by leveraging GPU direct storage. See [their GitHub repository](https://github.com/foundation-model-stack/fastsafetensors) for more details.
-For enabling this feature, set the environment variable ``USE_FASTSAFETENSOR`` to ``true``
+
+To enable this feature, use the ``--load-format fastsafetensors`` command-line argument