fix regexes with escape sequence
This PR fixes:

```
src/transformers/dynamic_module_utils.py:81
  /workspace/transformers/src/transformers/dynamic_module_utils.py:81: DeprecationWarning: invalid escape sequence \s
    relative_imports = re.findall("^\s*import\s+\.(\S+)\s*$", content, flags=re.MULTILINE)
src/transformers/dynamic_module_utils.py:83
  /workspace/transformers/src/transformers/dynamic_module_utils.py:83: DeprecationWarning: invalid escape sequence \s
    relative_imports += re.findall("^\s*from\s+\.(\S+)\s+import", content, flags=re.MULTILINE)
src/transformers/dynamic_module_utils.py:125
  /workspace/transformers/src/transformers/dynamic_module_utils.py:125: DeprecationWarning: invalid escape sequence \s
    imports = re.findall("^\s*import\s+(\S+)\s*$", content, flags=re.MULTILINE)
src/transformers/dynamic_module_utils.py:127
  /workspace/transformers/src/transformers/dynamic_module_utils.py:127: DeprecationWarning: invalid escape sequence \s
    imports += re.findall("^\s*from\s+(\S+)\s+import", content, flags=re.MULTILINE)
src/transformers/modeling_utils.py:222
  /workspace/transformers/src/transformers/modeling_utils.py:222: DeprecationWarning: invalid escape sequence \d
    bit_search = re.search("[^\d](\d+)$", str(dtype))
```

@sgugger 
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