We are currently solving the following issue within our repository. Here is the issue text:
--- BEGIN ISSUE ---
Create encoding preprocessors not until runtime
Right now the preprocessors are created at import

https://github.com/pmeier/pystiche/blob/cad5ab6e9485680f2543cf4397d0d21e72a88b9e/pystiche/enc/preprocessing.py#L1-L4

We should only create them if they are needed to speed up the import.

--- END ISSUE ---

Below are some code segments, each from a relevant file. One or more of these files may contain bugs.
--- BEGIN FILES ---
Path: `pystiche/enc/preprocessing.py`
Content:
```
1 from torch import nn
2 from pystiche.image import TorchPreprocessing, CaffePreprocessing
3 
4 PREPROCESSORS = {"torch": TorchPreprocessing(), "caffe": CaffePreprocessing()}
5 
6 __all__ = ["get_preprocessor"]
7 
8 
9 def get_preprocessor(framework: str) -> nn.Module:
10     return PREPROCESSORS[framework]
11
```
--- END FILES ---

Please first localize the bug based on the issue statement, and then generate a patch according to the `git diff` format fenced by three backticks.

Here is an example:

```diff
diff --git a/examples/server_async.py b/examples/server_async.py
--- a/examples/server_async.py
+++ b/examples/server_async.py
@@ -313,4 +313,4 @@
 
 
 if __name__ == "__main__":
-    asyncio.run(run_async_server("."), debug=True)
+    asyncio.run(run_async_server(), debug=True)
diff --git a/examples/server_sync.py b/examples/server_sync.py
--- a/examples/server_sync.py
+++ b/examples/server_sync.py
@@ -313,5 +313,5 @@
 
 
 if __name__ == "__main__":
-    server = run_sync_server(".")
+    server = run_sync_server()
     server.shutdown()

```

diff --git a/pystiche/enc/preprocessing.py b/pystiche/enc/preprocessing.py
--- a/pystiche/enc/preprocessing.py
+++ b/pystiche/enc/preprocessing.py
@@ -1,10 +1,10 @@
 from torch import nn
 from pystiche.image import TorchPreprocessing, CaffePreprocessing
 
-PREPROCESSORS = {"torch": TorchPreprocessing(), "caffe": CaffePreprocessing()}
+PREPROCESSORS = {"torch": TorchPreprocessing, "caffe": CaffePreprocessing}
 
 __all__ = ["get_preprocessor"]
 
 
 def get_preprocessor(framework: str) -> nn.Module:
-    return PREPROCESSORS[framework]
+    return PREPROCESSORS[framework]()

{"golden_diff": "diff --git a/pystiche/enc/preprocessing.py b/pystiche/enc/preprocessing.py\n--- a/pystiche/enc/preprocessing.py\n+++ b/pystiche/enc/preprocessing.py\n@@ -1,10 +1,10 @@\n from torch import nn\n from pystiche.image import TorchPreprocessing, CaffePreprocessing\n \n-PREPROCESSORS = {\"torch\": TorchPreprocessing(), \"caffe\": CaffePreprocessing()}\n+PREPROCESSORS = {\"torch\": TorchPreprocessing, \"caffe\": CaffePreprocessing}\n \n __all__ = [\"get_preprocessor\"]\n \n \n def get_preprocessor(framework: str) -> nn.Module:\n-    return PREPROCESSORS[framework]\n+    return PREPROCESSORS[framework]()\n", "issue": "Create encoding preprocessors not until runtime\nRight now the preprocessors are created at import\r\n\r\nhttps://github.com/pmeier/pystiche/blob/cad5ab6e9485680f2543cf4397d0d21e72a88b9e/pystiche/enc/preprocessing.py#L1-L4\r\n\r\nWe should only create them if they are needed to speed up the import.\n", "before_files": [{"content": "from torch import nn\nfrom pystiche.image import TorchPreprocessing, CaffePreprocessing\n\nPREPROCESSORS = {\"torch\": TorchPreprocessing(), \"caffe\": CaffePreprocessing()}\n\n__all__ = [\"get_preprocessor\"]\n\n\ndef get_preprocessor(framework: str) -> nn.Module:\n    return PREPROCESSORS[framework]\n", "path": "pystiche/enc/preprocessing.py"}], "after_files": [{"content": "from torch import nn\nfrom pystiche.image import TorchPreprocessing, CaffePreprocessing\n\nPREPROCESSORS = {\"torch\": TorchPreprocessing, \"caffe\": CaffePreprocessing}\n\n__all__ = [\"get_preprocessor\"]\n\n\ndef get_preprocessor(framework: str) -> nn.Module:\n    return PREPROCESSORS[framework]()\n", "path": "pystiche/enc/preprocessing.py"}]}