[BUG] Vmap call in value estimate not compatible with torch_geometric radius graph

The vmap call to in the value estimators is giving rise to an incompatibility issue.

In particular, this is the call
https://github.com/pytorch/rl/blob/98b45a6132b0094e100ec047ea73ec41eb52c078/torchrl/objectives/value/advantages.py#L144

My problem today is in relation to a feature for GNNs.

Basically in GNN you might need to compute the graph adjacency from an input vector containing positions using this function
https://pytorch-geometric.readthedocs.io/en/latest/generated/torch_geometric.nn.pool.radius_graph.html

This function, given inputs of the same shape, could give outputs of different size and was not made to be vmap compatible.

Here is the closest reprod script I came up with

import torch
from torch_geometric.nn import radius_graph


class Model(torch.nn.Module):
    def __init__(self):
        super().__init__()

    def forward(self, x, batch):
        edge_index = radius_graph(x, r=3.5, batch=batch, loop=False)
        return x


model = Model()

x = torch.tensor([[-1.0, -1.0], [-1.0, 1.0], [1.0, -1.0], [1.0, 1.0], [0.5, 0.5]])
batch = torch.tensor([0, 0, 1, 1, 1])

# Normal call works
data_out = model(x, batch)

# Vmap does not
x = torch.stack([x, x], dim=0)
batch = torch.stack([batch, batch], dim=0)
data_out = torch.vmap(model, (0, 0))(x, batch)

Any suggestions?

In general I think that this specific vmap call has been cause of incompatibility for many, would it be possible to make it optional?