Multiple buffer resize calls made in ParametricBuffer

Hi,
I'm working with ReplayBuffers and noticed that these methods are extremely slow. On digging into the code I noticed that the `update` function in this class calls `resize` 3 times for a single update, and since this involves going through the entire data, it ends up with a much longer runtime. The fix should be fairly simple.

Relevant Code snippets:
lines 355 - 373 of storage_policy.py
```
# update buffers with new data
for group_id, new_data_g in new_groups.items():
ll = group_to_len[group_id]
if group_id in self.buffer_groups:
old_buffer_g = self.buffer_groups[group_id]
old_buffer_g.update_from_dataset(strategy, new_data_g) ### Call 1
old_buffer_g.resize(strategy, ll) ### Call 2
else:
new_buffer = _ParametricSingleBuffer(
ll, self.selection_strategy
)
new_buffer.update_from_dataset(strategy, new_data_g)
self.buffer_groups[group_id] = new_buffer

# resize buffers
for group_id, class_buf in self.buffer_groups.items():
self.buffer_groups[group_id].resize(
strategy, group_to_len[group_id]
) ### Call 3
```

`update_from_dataset` function calls resize within its call - lines 440-442
```
def update_from_dataset(self, strategy, new_data):
self.buffer = AvalancheConcatDataset([self.buffer, new_data])
self.resize(strategy, self.max_size)
```