Repository: deepset-ai/haystack

Issue:
Importing Haystack with Windows Fails due to FAISS
I cannot import haystack on Windows

```
Traceback (most recent call last):
  File "C:/Users/Utilisateur/PythonProjects/piaf-ml/src/evaluation/retriever_reader/retriever_reader_eval_squad.py", line 10, in <module>
    from haystack.document_store.elasticsearch import ElasticsearchDocumentStore
  File "c:\users\utilisateur\pythonprojects\haystack\haystack\__init__.py", line 5, in <module>
    from haystack.finder import Finder
  File "c:\users\utilisateur\pythonprojects\haystack\haystack\finder.py", line 8, in <module>
    from haystack.reader.base import BaseReader
  File "c:\users\utilisateur\pythonprojects\haystack\haystack\reader\__init__.py", line 1, in <module>
    from haystack.reader.farm import FARMReader
  File "c:\users\utilisateur\pythonprojects\haystack\haystack\reader\farm.py", line 24, in <module>
    from haystack.document_store.base import BaseDocumentStore
  File "c:\users\utilisateur\pythonprojects\haystack\haystack\document_store\__init__.py", line 2, in <module>
    from haystack.document_store.faiss import FAISSDocumentStore
  File "c:\users\utilisateur\pythonprojects\haystack\haystack\document_store\faiss.py", line 17, in <module>
    raise ModuleNotFoundError("FAISSDocumentStore on windows platform is not supported")
ModuleNotFoundError: FAISSDocumentStore on windows platform is not supported
```

I think this is due to the PR #800 where the __init__ file for Document_Store was modified. At least, the error disappears when i checkout to commit f3a3b73d9b7a0446a17f48dd398449677521eeb9 

**System:**
 - OS: Window
 - GPU/CPU: GPU
 - Haystack version (commit or version number): e91518ee00f2cab0fc10c4741c775d2fb5a4a1cf
 - DocumentStore: ElasticSearch 




Discussion:
Hi @Rob192, can you share the output of the following code on your Python shell? 

```python
import sys
print(sys.platform)
```

The check [here](https://github.com/deepset-ai/haystack/blob/e91518ee00f2cab0fc10c4741c775d2fb5a4a1cf/haystack/document_store/faiss.py#L14) should not have raised an exception on Windows.
Hi, it prints :
```
win32
```
@tanaysoni I think @Rob192 is talking about change in __init.py file. https://github.com/deepset-ai/haystack/blob/e91518ee00f2cab0fc10c4741c775d2fb5a4a1cf/haystack/document_store/__init__.py

As this file have following import -
```
from haystack.document_store.faiss import FAISSDocumentStore
```
So even though user don't want to use FAISSDocumentStore this import will trigger ModuleNotFound exception on windows platform due to this https://github.com/deepset-ai/haystack/blob/e91518ee00f2cab0fc10c4741c775d2fb5a4a1cf/haystack/document_store/faiss.py#L14

Anyway I think faiss is now supported on Windows we can remove this exception.
Thanks @lalitpagaria for making my point clearer

Resolving patch:
diff --git a/haystack/document_store/faiss.py b/haystack/document_store/faiss.py
--- a/haystack/document_store/faiss.py
+++ b/haystack/document_store/faiss.py
@@ -1,8 +1,8 @@
 import logging
-from sys import platform
 from pathlib import Path
 from typing import Union, List, Optional, Dict, Generator
 from tqdm import tqdm
+import faiss
 import numpy as np
 
 from haystack import Document
@@ -11,10 +11,6 @@
 from haystack.utils import get_batches_from_generator
 from scipy.special import expit
 
-if platform != 'win32' and platform != 'cygwin':
-    import faiss
-else:
-    raise ModuleNotFoundError("FAISSDocumentStore on windows platform is not supported")
 
 logger = logging.getLogger(__name__)
 


Test patch: