[`bnb`] Fix bnb slow test
# What does this PR do?

Fixes a failing daily CI test. https://github.com/huggingface/transformers/actions/runs/4485738346/jobs/7887641218

Which can be simplified as:
```python
from transformers import AutoModelForCausalLM

model_name = "bigscience/bloom-560m"

memory_mapping = {0: "1GB", 1: "2GB"}
model_parallel = AutoModelForCausalLM.from_pretrained(
    model_name, load_in_8bit=True, max_memory=memory_mapping, device_map="auto"
)

# Check correct device map
print(set(model_parallel.hf_device_map.values()))
>>> EXPECTED={0, 1} / GOT={0}
```

I think this also fixes a bug (which is not negligeable)

It seems that we need to add a check `max_memory is None` otherwise `max_memory` will get overriden right after by
```python
max_memory = get_balanced_memory(
                    model,
                    dtype=torch_dtype,
                    low_zero=(device_map == "balanced_low_0"),
                    **kwargs,
                )
```
Hence `max_memory` seems to be ignored now on the `main` branch in some cases

cc @sgugger @ydshieh 
_The documentation is not available anymore as the PR was closed or merged._<|||||>Seems to be fixed on `main`, probably thanks to https://github.com/huggingface/transformers/pull/22311 