Promptless segmentation

Hi all,
I am trying to build a pipeline to train **without prompts and only use the default sparse & dense embeddings + image embedding**. For some reason, the resulting segmentation doesn't seem to do well compared to the demos. Please note that this code is different from the inference code provided in the repository because my ultimate intention is to train the model. Also, I am using the approach discussed in #138 to input flexible image size.

```
def iteration(predictor, batch):
# process WITHOUT any prompts

batched_mode = True if batch["mask"].ndim == 4 else False # trying no batch first
if batched_mode:
predictor.set_image_batch(batch["image"]) # apply SAM image encodet to the image, is it automatically normalized
else:
predictor.set_image(batch["image"]) # apply SAM image encodet to the image, is it automatically normalized

sparse_embeddings, dense_embeddings = predictor.model.sam_prompt_encoder(points=None,boxes=None,masks=None)

low_res_masks, prd_scores, _, _ = predictor.model.sam_mask_decoder(
image_embeddings=predictor._features["image_embed"][-1].unsqueeze(0),
image_pe=predictor.model.sam_prompt_encoder.get_dense_pe(),
sparse_prompt_embeddings=sparse_embeddings,
dense_prompt_embeddings=dense_embeddings,
multimask_output=True,
repeat_image=batched_mode,
high_res_features=None
)

# ! resolve orig_hw thing
prd_masks = predictor._transforms.postprocess_masks(low_res_masks, predictor._orig_hw[-1])

print("prd_masks", prd_masks.shape, low_res_masks.shape)

prd_mask = torch.sigmoid(prd_masks[:, 0]) > 0.5
# prd_mask = torch.sigmoid(prd_masks) > 0.5

print("final prd masks shape", prd_mask.shape)

return prd_mask # (channel, img_size, img_size)
```

Please see this screenshot of the resulting segmentation mask overlaid on top of the original image.


Thanks for the help!