Make `_LazyAutoMapping` a bit less lazy 😪 🔥 
# What does this PR do?

Current `_LazyAutoMapping` is a bit too lazy, and potentially to give silent failure/error. For example, tests being skipped by mistake. One explicit example is
https://github.com/huggingface/transformers/blob/c26b1503f22feb20d37588d4243bd444d297bc13/tests/test_pipeline_common.py#L86
(so far in a not-yet merged PR), which was previously
```python
if model_mapping is None or len(model_mapping) == 0: 
```
and the tests were all skipped, and I am like 😕 .

As a former mathematician, it's hard for me to accept length being 0 in this case 🔢 . I decide to make it less lazy!

## Results

### Before this PR
```python
from transformers import TF_MODEL_MAPPING

print(len(TF_MODEL_MAPPING)) # 0
```

### After this PR
```after
from transformers import TF_MODEL_MAPPING

print(len(TF_MODEL_MAPPING))  # 60
```
_The documentation is not available anymore as the PR was closed or merged._<|||||>Failing tests are irrelevant.