[Installation] Avoid ModuleNotFoundError:setuptools-scm error (#475)

Current instructions for setup using standalone docker (not using
Dockerfile) is missing the `pip install -r requirements-hpu.txt`
instruction. A new user using this method for setup will encounter :
```
File "/root/vllm/setup.py", line 15, in <module>
    from setuptools_scm import get_version
ModuleNotFoundError: No module named 'setuptools_scm'
```
This PR fixes that.

docs/source/getting_started/gaudi-installation.rst

@@ -75,6 +75,7 @@ To build and install vLLM from source, run:
 
    $ git clone https://github.com/vllm-project/vllm.git
    $ cd vllm
+   $ pip install -r requirements-hpu.txt
    $ python setup.py develop
 
 
@@ -85,6 +86,7 @@ Currently, the latest features and performance optimizations are developed in Ga
    $ git clone https://github.com/HabanaAI/vllm-fork.git
    $ cd vllm-fork
    $ git checkout habana_main
+   $ pip install -r requirements-hpu.txt
    $ python setup.py develop