After some digging around, it would appear that although in the Github repo the hardcoding was fixed, when conda installing the stable release of fastai the problem is still there
So to fix this problem you can temporarily monkey patch it this way:
def _one_hidden(self, l:int)->Tensor:
nh = (self.n_hid if l != self.n_layers - 1 else self.emb_sz) // self.n_dir
return one_param(self).new(self.n_dir, self.bs, nh).zero_()
learn.model[0]._one_hidden = partial( _one_hidden, learn.model[0])
After which you can LR_find as usual and train your model.