Update gpuhunt contributing guide (#4143)

Author: r4victor

contributing/BACKENDS.md

@@ -31,14 +31,23 @@ git clone https://github.com/dstackai/gpuhunt.git
 - **Online providers** offer dynamic machine configurations that are available at the very moment
   when you fetch configurations (e.g., GPU marketplaces).
   `gpuhunt` collects online providers' instance offers each time a `dstack` user provisions a new instance.
-  Examples: `tensordock`, `vastai`, etc.
+  Examples: `vastai`, `hotaisle`, etc.
 
 ### 1.3. Create the provider class
 
 Create the provider class file under `src/gpuhunt/providers`.
 
-Make sure your class extends the [`AbstractProvider`](https://github.com/dstackai/gpuhunt/blob/main/src/gpuhunt/providers/__init__.py)
-base class. See its docstrings for descriptions of the methods that your class should implement.
+Make sure your class extends either `OnlineProvider` or `OfflineProvider` from
+[base.py](https://github.com/dstackai/gpuhunt/blob/main/src/gpuhunt/providers/base.py),
+matching the choice you made above.
+
+Both kinds implement `get`, returning `CatalogItem`s with `provider` set to your provider's `NAME`.
+Additionally:
+
+- Online providers implement the `from_env` classmethod, which reads credentials from the
+  environment with `get_creds_env` and raises `MissingCredsError` if one is missing. Providers that
+  raise it are skipped by `default_catalog()` rather than failing the whole catalog.
+- Offline providers may override `filter` to omit some offers from the published catalog.
 
 Refer to examples:
 - Offline providers:
@@ -47,8 +56,8 @@ Refer to examples:
   [azure.py](https://github.com/dstackai/gpuhunt/blob/main/src/gpuhunt/providers/azure.py),
   [lambdalabs.py](https://github.com/dstackai/gpuhunt/blob/main/src/gpuhunt/providers/lambdalabs.py).
 - Online providers:
-  [vultr.py](https://github.com/dstackai/gpuhunt/blob/main/src/gpuhunt/providers/vultr.py)
-  [tensordock.py](https://github.com/dstackai/gpuhunt/blob/main/src/gpuhunt/providers/tensordock.py),
+  [vultr.py](https://github.com/dstackai/gpuhunt/blob/main/src/gpuhunt/providers/vultr.py),
+  [hotaisle.py](https://github.com/dstackai/gpuhunt/blob/main/src/gpuhunt/providers/hotaisle.py),
   [vastai.py](https://github.com/dstackai/gpuhunt/blob/main/src/gpuhunt/providers/vastai.py).
 
 ### 1.4. Register the provider with the catalog
@@ -57,7 +66,8 @@ Add your provider in the following places:
 - Either `OFFLINE_PROVIDERS` or `ONLINE_PROVIDERS` in `src/gpuhunt/_internal/catalog.py`.
 - The `python -m gpuhunt` command in `src/gpuhunt/__main__.py`.
 - (offline providers) The CI workflow in `.github/workflows/catalogs.yml`.
-- (online providers) The default catalog in `src/gpuhunt/_internal/default.py`.
+- (online providers) `ONLINE_PROVIDER_MODULES` in `src/gpuhunt/_internal/default.py`, which is what
+  `default_catalog()` loads.
 
 ### 1.5. Add data quality tests
 

contributing/GPUHUNT.md

@@ -4,22 +4,27 @@
 
 An offer is a possible configuration. It consists of:
 - Provider (or backend in dstack)
+- CPU architecture
 - CPU count
 - RAM size
 - Disk size
 - GPU count
+- GPU vendor (if any)
 - GPU model name (if any)
 - GPU VRAM size (if any)
 - Is interruptible (or spot)
 - Region (provider-specific)
 - Instance name or ID (provider-specific)
 - Price per hour
 
+Offers are represented by `CatalogItem`. Providers construct them directly, setting `provider` to
+their own `NAME`, and `gpu_vendor` whenever `gpu_count` is non-zero.
+
 ## Catalog
 
 Some providers don't have a suitable API for querying all offers in real-time. That's why gpuhunt has two types of providers:
 
-- Online — offers can be queried in real-time
+- Online — offers can be queried quickly in real-time
 - Offline — offers must be loaded from a precomputed catalog file
 
 The `Catalog` class hides those details from the user, reading offers from the file for offline providers or querying online providers.
@@ -28,7 +33,16 @@ The `Catalog` class pulls the latest catalog from the S3 bucket and caches it fo
 
 ## Provider implementation
 
-Providers must implement a single method `get`. It has the same name for both online and offline providers but works differently.
+Providers subclass either `OnlineProvider` or `OfflineProvider` from `src/gpuhunt/providers/base.py`.
+
+Both implement `get`. It has the same name for both online and offline providers but works differently.
+
+In addition:
+
+- Online providers implement the `from_env` classmethod, since `default_catalog()` constructs them
+  in the user's process.
+- Offline providers may override `filter` to omit some offers from the published catalog.
+  Credentials are passed in by the caller, so a missing one is an error rather than a skip.
 
 ### Offers sorting
 
@@ -100,7 +114,7 @@ These mechanisms are used to preserve backward compatibility:
 
 - **`gpuhunt` version**: The interfaces in the `gpuhunt` package preserve backward compatibility
   within a minor version (`X` in `0.X.Y`).
-- **Offer flags**: If an offer breaks older `dstack` versions, it is marked with a flag in `RawCatalogItem.flags`
+- **Offer flags**: If an offer breaks older `dstack` versions, it is marked with a flag in `CatalogItem.flags`
   and the flag is added to the list of supported flags in `dstack`.
   Older `dstack` versions that don't support this flag will not see the respective offers.
 - **Offline catalog versions**: If a breaking change in the structure or content of an offline catalog is unavoidable,