fix(server): llama v2 GPTQ (#648)

As per title & reported
huggingface/text-generation-inference#601 (comment)
https://huggingface.co/TheBloke/Llama-2-70B-chat-GPTQ/discussions/5

Test it:

```
GPTQ_BITS=4 GPTQ_GROUPSIZE=1 text-generation-launcher --model-id TheBloke/Llama-2-70B-chat-GPTQ --port 8080 --num-shard 4 --quantize gptq
```
&
```
curl 127.0.0.1:8080/generate \
    -X POST \
    -d '{"inputs":"hey llama","parameters":{"max_new_tokens":256}}' \
    -H 'Content-Type: application/json'
```

server/text_generation_server/models/custom_modeling/flash_llama_modeling.py

@@ -148,24 +148,27 @@ def forward(self, hidden_states, residual=None):
 
 
 def _load_gqa(config, prefix: str, weights):
-    w = [
-        weights.get_sharded(f"{prefix}.q_proj.weight", dim=0),
-        weights.get_sharded(f"{prefix}.k_proj.weight", dim=0),
-        weights.get_sharded(f"{prefix}.v_proj.weight", dim=0),
-    ]
-    weight = torch.cat(w, dim=0)
-    weight = weight.to(dtype=weights.dtype).to(device=weights.device)
-    bias = None
     assert config.hidden_size % config.num_attention_heads == 0
-    head_size = config.hidden_size // config.num_attention_heads
     assert config.num_attention_heads % weights.process_group.size() == 0
-    num_heads = config.num_attention_heads // weights.process_group.size()
-    num_key_value_heads = config.num_key_value_heads // weights.process_group.size()
-    assert list(weight.shape) == [
-        (num_heads + 2 * num_key_value_heads) * head_size,
-        config.hidden_size,
-    ], f"{list(weight.shape)} != {[(num_heads + 2 * config.num_key_value_heads) * head_size, config.hidden_size]}"
-    return TensorParallelColumnLinear(get_linear(weight, bias, config.quantize))
+
+    weight = weights.get_multi_weights_col(
+        prefixes=[f"{prefix}.q_proj", f"{prefix}.k_proj", f"{prefix}.v_proj"],
+        quantize=config.quantize,
+        dim=0
+    )
+
+    if config.quantize != "gptq":
+        weight = weight.to(dtype=weights.dtype).to(device=weights.device)
+
+        head_size = config.hidden_size // config.num_attention_heads
+        num_heads = config.num_attention_heads // weights.process_group.size()
+        num_key_value_heads = config.num_key_value_heads // weights.process_group.size()
+        assert list(weight.shape) == [
+            (num_heads + 2 * num_key_value_heads) * head_size,
+            config.hidden_size,
+        ], f"{list(weight.shape)} != {[(num_heads + 2 * config.num_key_value_heads) * head_size, config.hidden_size]}"
+
+    return TensorParallelColumnLinear(get_linear(weight, bias=None, quantize=config.quantize))
 
 
 class FlashLlamaAttention(torch.nn.Module):