what does colon wrapped class mean in python comment?
What does colon wrapped class (:class:) mean in python comment?

For example,

class Optimizer(object):
    r"""Base class for all optimizers.
    Arguments:
        params (iterable): an iterable of :class:`torch.Tensor` s or
            :class:`dict` s. Specifies what Tensors should be optimized.
        defaults: (dict): a dict containing default values of optimization
            options (used when a parameter group doesn't specify them).
    """

    def __init__(self, params, defaults):
        self.defaults = defaults


Is it a pytorch-specific syntax, or what?

Source: https://github.com/pytorch/pytorch/blob/master/torch/optim/optimizer.py

It's nothing Python or Torch specific. It's syntax for a documentation tool; most likely Sphinx. 

The syntax indicates a cross-reference to the documentation for another class. When the documentation tool generates hyperlinked output such as HTML, such references automatically become links to the documentation page for the named class.

For Sphinx, see the Cross-referencing syntax documentation; there you'd see py:class: as a class-reference, but we can assume the default domain is set to Python so :class: is valid too.

The PyTorch project indeed uses Sphinx to generate the documenation. You can compare the source you found withthe resulting generated documenation; note how both dict and torch.Tensor are hyperlinks to more documentation.
