Add inference quickstart guide

docs/inference_quickstart.md

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+## Inference Quickstart
+
+This quickstart shows how to (1) initialize the OlmoEarth model, (2) obtain a satellite
+image suitable for input to the model, and (3) compute embeddings from that satellite
+image.
+
+## Initializing the Model
+
+First, setup a Python 3.12 environment. You can use your favorite Python package
+manager, but here is an example with uv:
+
+```
+curl -LsSf https://astral.sh/uv/install.sh | sh
+cd /path/to/olmoearth_pretrain/
+uv sync
+```
+
+Now we can use the `olmoearth_pretrain` library to initialize the model in pytorch.
+Below, we initialize the OlmoEarth-v1-Base model.
+
+```python
+from olmoearth_pretrain.model_loader import ModelID, load_model
+model = load_model(ModelID.OLMOEARTH_V1_BASE)
+```
+
+## Obtain Satellite Imagery
+
+Here, we obtain one Sentinel-2 image from the ESA Copernicus Browser. If you want to
+apply the model on multiple images of a location, like a time series of Sentinel-1 and
+Sentinel-2 images, see the
+[OlmoEarth embedding](https://github.com/allenai/rslearn/blob/master/docs/examples/OlmoEarthEmbeddings.md).
+and [OlmoEarth fine-tuning](https://github.com/allenai/rslearn/blob/master/docs/examples/FinetuneOlmoEarth.md)
+guides in rslearn.
+
+To download on image from the Copernicus Browser, follow these steps:
+
+1. Navigate to https://browser.dataspace.copernicus.eu/. Press Login to sign up for an
+   account and login.
+2. Go to the Search tab at the top-left. Check Sentinel-2, then check L2A. This selects
+   Sentinel-2 L2A images, which are the type of Sentinel-2 images that OlmoEarth is
+   pre-trained on.
+3. Modify the time range if desired. Also, use the area of interest tool at the top
+   right to select a spatial area to search over.
+4. Then, press Search. We recommend looking through the results to find a less cloudy
+   image. You can press Visualize to preview the satellite image in the Browser before
+   downloading it. Once you are satisfied, press the download icon next to the image in
+   the search results. Once the download is complete, unzip the file.
+
+If you prefer to skip using the browser, you can download and unzip a Sentinel-2 image
+of Seattle:
+
+```
+wget https://storage.googleapis.com/ai2-rslearn-projects-data/artifacts/example_sentinel2_l2a_scene_of_seattle.zip
+unzip example_sentinel2_l2a_scene_of_seattle.zip
+```
+
+## Compute Embeddings
+
+Finally, we load the image in Python, normalize it, and apply the model on it to
+compute embeddings.
+
+First, we read all of the image bands at 10 m/pixel. We use the B02 band (one of the
+10 m/pixel bands) to determine the transform under which to read the remaining bands,
+since some are stored at lower resolutions. Note that here we assume that the .SAFE
+folder is in the working directory.
+
+```python
+import glob
+import numpy as np
+import rasterio
+from olmoearth_pretrain.data.constants import Modality
+from rasterio.vrt import WarpedVRT
+from rasterio.enums import Resampling
+
+# Get the JP2 filenames that we need to read, in the band order expected by OlmoEarth.
+fnames = []
+for band_name in Modality.SENTINEL2_L2A.band_order:
+    fname = glob.glob(f"*.SAFE/GRANULE/*/IMG_DATA/*/*_{band_name}_*.jp2")[0]
+    fnames.append(fname)
+
+# Get the CRS and transform from the first band, which is B02.
+with rasterio.open(fnames[0]) as src:
+    crs = src.crs
+    transform = src.transform
+    width = src.width
+    height = src.height
+
+# We limit the width/height to 512x512 in case there is limited memory.
+width = 512
+height = 512
+
+# Now read all of the bands.
+image = np.zeros((len(fnames), height, width), dtype=np.int32)
+for band_idx, fname in enumerate(fnames):
+    with rasterio.open(fname) as src:
+        with rasterio.vrt.WarpedVRT(
+            src,
+            crs=crs,
+            transform=transform,
+            width=width,
+            height=height,
+            resampling=Resampling.bilinear,
+        ) as vrt:
+            image[band_idx, :, :] = vrt.read(1)
+
+# Rearrange to BHWTC.
+image = image.transpose(1, 2, 0)[None, :, :, None, :]
+```
+
+Next, we normalize the image:
+
+```python
+from olmoearth_pretrain.data.normalize import Normalizer, Strategy
+
+normalizer = Normalizer(Strategy.COMPUTED)
+image = normalizer.normalize(Modality.SENTINEL2_L2A, image)
+```
+
+Now we can apply the model on the image. We recommend applying it on inputs between
+1x1 and 128x128 in size. The patch size can be set between 1 and 8; smaller patch sizes
+generally perform better, but require more GPU time.
+
+```python
+import torch
+from olmoearth_pretrain.train.masking import MaskedOlmoEarthSample, MaskValue
+
+device = torch.device("cuda")
+model.to(device)
+
+# Run the model on the topleft 64x64 of the image.
+sample = MaskedOlmoEarthSample(
+    sentinel2_l2a=torch.tensor(image[:, 0:64, 0:64, :, :], dtype=torch.float32, device=device),
+    # The mask shape is BHWTS, where S is the number of band sets (3 for Sentinel-2).
+    sentinel2_l2a_mask=torch.ones((1, 64, 64, 1, 3), dtype=torch.float32, device=device) * MaskValue.ONLINE_ENCODER.value,
+    # The timestamps is (day of month 1-31, month 0-11, year).
+    # The values here correspond to the date of our sample Sentinel-2 image of Seattle
+    # (2025-08-22).
+    timestamps=torch.tensor([22, 7, 2025], device=device)[None, None, :],
+)
+tokens_and_masks = model.encoder(
+    sample, fast_pass=True, patch_size=4,
+)["tokens_and_masks"]
+# Get the Sentinel-2 features.
+modality_features = tokens_and_masks.sentinel2_l2a
+# Pool the features over the timestep and band set dimensions so we end up with a BHWC
+# feature map.
+pooled = modality_features.mean(dim=[3, 4])
+```