Why does nnx.Rngs allow default to be none?

[Melody-coder923]

nnx.Rngs() is allowed and default param defaults to None but is this actually ever valid? Because most modules try to get stream for params or atleast a stream. if its None, no stream is created. Function signature shows default can be None . I am curious. Does it make sense to add warning or not allow default=None.?

Function signature

 The ``default`` stream can be created by passing in a key to the constructor without
  specifying a stream name. When the ``default`` stream is set the ``rngs`` object can be
  called directly to get a key, and calling streams that were not specified during
  construction will fallback to ``default``::

    >>> rngs = nnx.Rngs(0, params=1)
    ...
    >>> key1 = rngs.default()       # uses 'default'
    >>> key2 = rngs()               # uses 'default'
    >>> key3 = rngs.params()        # uses 'params'
    >>> key4 = rngs.dropout()       # uses 'default'
    >>> key5 = rngs.unkown_stream() # uses 'default'
    ```

[vfdev-5]

@Melody-coder923 thanks for the question. I'll ask other team members as I'm not totally sure why default=None is supported. Once I get the answer I'll get back to you.