diff --git a/activation_store/collect.py b/activation_store/collect.py
index 77a3b30..5cb54e2 100644
--- a/activation_store/collect.py
+++ b/activation_store/collect.py
@@ -14,6 +14,8 @@ from transformers.modeling_outputs import ModelOutput
 from activation_store.helpers.torch import clear_mem
 from typing import Dict, Generator, List, Union, Optional
 from torch import Tensor
+import tempfile
+import gc
 import os
 
 default_output_folder = (Path(__file__).parent.parent / "outputs").resolve()
@@ -35,17 +37,18 @@ def default_postprocess_result(input: dict, trace: TraceDict, output: ModelOutpu
         for k, group in act_groups.items():
             aas = [v.output[0] if isinstance(v.output, tuple) else v.output for k, v in trace.items() if k in group]
             assert len(aas) > 0, f"no activations found for {group}"
+            assert aas[0].dim() == 3, f"expected [b, t, h] activations, got {aas[0].shape}"
             aas = torch.stack([a[:, token_index].to(dtype) for a in aas], dim=1)
             acts[f'acts-{k}'] = aas
     else:
-        acts = {f'act-{k}': 
+        acts = {f'acts-{k}': 
                 v.output[0] if isinstance(v.output, tuple) else v.output
                 for k, v in trace.items()}
         acts = {k: v[:, token_index].to(dtype) for k, v in acts.items() if v is not None}
     del trace
     
     # batch must be first, also the writer supports float16 so lets use that
-    output.hidden_states = rearrange(list(output.hidden_states), 'l b t h -> b l t h')[:, :, token_index].to(dtype)
+    output.hidden_states = rearrange([h[:, token_index] for h in output.hidden_states], 'l b t h -> b l t h').to(dtype)
 
     output.logits = output.logits[:, token_index].to(dtype)
 
@@ -60,6 +63,12 @@ def default_postprocess_result(input: dict, trace: TraceDict, output: ModelOutpu
         if v.dim() == 0:
             bs = input['input_ids'].shape[0]
             o[k] = v.repeat(bs)
+
+
+    # finally check for nans
+    for k, v in o.items():
+        if torch.isnan(v).any():
+            raise ValueError(f"nan found in {k}")
     return o
 
 
@@ -91,8 +100,12 @@ def generate_batches(loader: DataLoader, model: AutoModelForCausalLM, layers = [
 
             # FIXME for some reason autocast isn't converting the inputs
             batch = {k: v.to(device) if isinstance(v, Tensor) else v for k, v in batch.items()}
-            with TraceDict(model, layers, retain_grad=False, detach=True, clone=True) as trace:
+            if layers is not None:
+                with TraceDict(model, layers, retain_grad=False, detach=True, clone=True) as trace:
+                    out = model(**batch, use_cache=False, output_hidden_states=True, return_dict=True)
+            else:
                 out = model(**batch, use_cache=False, output_hidden_states=True, return_dict=True)
+                trace = None
         o = postprocess_result(batch, trace, out, model, act_groups=act_groups)
 
         # copy to avoid memory leaks
@@ -108,7 +121,7 @@ def dataset_hash(**kwargs):
     return suffix
 
 
-def activation_store(loader: DataLoader, model: AutoModelForCausalLM, dataset_name='', layers: Union[List[str], Dict[str, List[str]]]=[], dataset_dir=default_output_folder, writer_batch_size=1, postprocess_result=default_postprocess_result) -> Dataset:
+def activation_store(loader: DataLoader, model: AutoModelForCausalLM, dataset_name='', layers: Union[List[str], Dict[str, List[str]]]=[], dataset_dir=default_output_folder, writer_batch_size=1, postprocess_result=default_postprocess_result, outfile: Optional[Path] = None) -> Dataset:
     """
     Collect activations from a model and store them in a dataset
 
@@ -130,13 +143,16 @@ def activation_store(loader: DataLoader, model: AutoModelForCausalLM, dataset_na
         Dataset.from_parquet(f).with_format("torch")
     """
     hash = dataset_hash(generate_batches=generate_batches, loader=loader, model=model)
-    f = dataset_dir / ".ds" / f"ds_{dataset_name}_{hash}.parquet"
-    f.parent.mkdir(exist_ok=True, parents=True)
-    logger.info(f"creating dataset {f}")
+
+    if outfile is None:
+        outdir = Path(tempfile.mkdtemp(prefix='activation_store'))
+        outfile = outdir / f"ds_{dataset_name}_{hash}.parquet"
+    outfile.parent.mkdir(exist_ok=True, parents=True)
+    logger.info(f"creating dataset {outfile}")
 
     iterator = generate_batches(loader, model, layers=layers, postprocess_result=postprocess_result) 
 
-    with ParquetWriter(path=f, writer_batch_size=writer_batch_size,
+    with ParquetWriter(path=outfile, writer_batch_size=writer_batch_size,
                        embed_local_files=True
                        ) as writer: 
         for bo in iterator:
@@ -146,7 +162,9 @@ def activation_store(loader: DataLoader, model: AutoModelForCausalLM, dataset_na
 
             # or maybe better compression to `writer.write(example, key)` for each
             writer.write_batch(bo)
+            del bo
+            gc.collect()
         writer.finalize() 
         writer.close()
     
-    return f
+    return outfile