Remove conditional import of FAISS for Windows

[tanaysoni]

Since FAISS is now supported on Windows, we can remove the conditional import of faiss in the FAISSDocumentDocumentStore.

Resolves #817

[tholor]

I gave it a quick test in a Windows VM. Installation of 1.6.3 via pip still seems to crash:

    Complete output (38 lines):
    running install
    running build
    running build_ext
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "C:\Users\PietschMalte\AppData\Local\Temp\pip-install-iue3tn7s\faiss-cpu\setup.py", line 87, in <module>
        setup(
      File "C:\Users\PietschMalte\Miniconda3\lib\site-packages\setuptools\__init__.py", line 144, in setup
        return distutils.core.setup(**attrs)
      File "C:\Users\PietschMalte\Miniconda3\lib\distutils\core.py", line 148, in setup
        dist.run_commands()
      File "C:\Users\PietschMalte\Miniconda3\lib\distutils\dist.py", line 966, in run_commands
        self.run_command(cmd)
      File "C:\Users\PietschMalte\Miniconda3\lib\distutils\dist.py", line 985, in run_command
        cmd_obj.run()
      File "C:\Users\PietschMalte\Miniconda3\lib\site-packages\setuptools\command\install.py", line 61, in run
        return orig.install.run(self)
      File "C:\Users\PietschMalte\Miniconda3\lib\distutils\command\install.py", line 545, in run
        self.run_command('build')
      File "C:\Users\PietschMalte\Miniconda3\lib\distutils\cmd.py", line 313, in run_command
        self.distribution.run_command(command)
      File "C:\Users\PietschMalte\Miniconda3\lib\distutils\dist.py", line 985, in run_command
        cmd_obj.run()
      File "C:\Users\PietschMalte\Miniconda3\lib\distutils\command\build.py", line 135, in run
        self.run_command(cmd_name)
      File "C:\Users\PietschMalte\Miniconda3\lib\distutils\cmd.py", line 313, in run_command
        self.distribution.run_command(command)
      File "C:\Users\PietschMalte\Miniconda3\lib\distutils\dist.py", line 985, in run_command
        cmd_obj.run()
      File "C:\Users\PietschMalte\AppData\Local\Temp\pip-install-iue3tn7s\faiss-cpu\setup.py", line 45, in run
        build_ext.run(self)
      File "C:\Users\PietschMalte\Miniconda3\lib\distutils\command\build_ext.py", line 340, in run
        self.build_extensions()
      File "C:\Users\PietschMalte\AppData\Local\Temp\pip-install-iue3tn7s\faiss-cpu\setup.py", line 50, in build_extensions
        self._remove_flag('-Wstrict-prototypes')
      File "C:\Users\PietschMalte\AppData\Local\Temp\pip-install-iue3tn7s\faiss-cpu\setup.py", line 58, in _remove_flag
        compiler = self.compiler.compiler
    AttributeError: 'MSVCCompiler' object has no attribute 'compiler'
    ----------------------------------------

1.6.5 and 1.7.0 seem to work (at least installation succeeded).
I would propose to update to faiss 1.7.0 and test if everything in Haystack is still compatible

[lalitpagaria]

Actually we can use 1.7.0 which have latest changes.

Also soon they will support M1 Mac support

[lalitpagaria]

Actually we can use 1.7.0 which have latest changes.

Also soon they will support M1 Mac support

Sorry I missed reading @tholor comment

[tholor]

@tanaysoni we'll need FAISS benchmarks here (which are not included yet in the CI runs). I can take care of this

[tholor]

Benchmarks look okay. mAP is similar. speed is a bit slower but can be due to smaller random fluctuations. I think the benefit of moving to the latest FAISS version is big enough to justify merging.

FAISS Benchmarks for this branch:

	retriever	doc_store	n_docs	n_queries	retrieve_time	queries_per_second	seconds_per_query	recall	map	top_k	date_time	error
0	dpr	faiss_flat	1000	1064	30.651	34.7133	0.0288074	99.1541	92.9511	10	2021-02-11 09:23:44.893784
2	dpr	faiss_flat	10000	5637	256.807	21.9503	0.0455574	97.4987	89.871	10	2021-02-11 09:29:55.871041
4	dpr	faiss_flat	100000	5637	1187.45	4.74715	0.210653	95.7956	86.5461	10	2021-02-11 09:57:26.376901
1	dpr	faiss_hnsw	1000	1064	30.4996	34.8857	0.028665	99.1541	92.9511	10	2021-02-11 09:24:47.604630
3	dpr	faiss_hnsw	10000	5637	193.939	29.0658	0.0344047	97.2503	89.6994	10	2021-02-11 09:34:11.557157
5	dpr	faiss_hnsw	100000	5637	503.186	11.2026	0.0892648	94.0216	85.0798	10	2021-02-11 10:11:33.449578

Last run on master:

	retriever	doc_store	n_docs	n_queries	retrieve_time	queries_per_second	seconds_per_query	recall	map	top_k	date_time	error
2	dpr	faiss_flat	1000	1064	30.0533	35.4038	0.0282456	0.991541	0.929511	10	2021-02-01 11:28:55.544474
6	dpr	faiss_flat	10000	5637	218.594	25.7875	0.0387785	0.974987	0.89871	10	2021-02-01 11:42:07.545869
10	dpr	faiss_flat	100000	5637	865.744	6.51116	0.153582	0.957956	0.865461	10	2021-02-01 12:34:29.493598
14	dpr	faiss_flat	500000	5637	3717.95	1.51616	0.659561	0.930814	0.808614	10	2021-02-01 16:12:52.804436
3	dpr	faiss_hnsw	1000	1064	27.1677	39.1641	0.0255336	0.991541	0.929511	10	2021-02-01 11:30:02.684535
7	dpr	faiss_hnsw	10000	5637	167.552	33.6432	0.0297237	0.972503	0.896994	10	2021-02-01 11:46:07.130588
11	dpr	faiss_hnsw	100000	5637	167.482	33.6573	0.0297112	0.940216	0.850798	10	2021-02-01 12:43:21.697968
15	dpr	faiss_hnsw	500000	5637	164.456	34.2767	0.0291743	0.882562	0.769148	10	2021-02-01 16:47:01.710072