chore: move transformer to operator-requirements.txt (#4089)

### What changes were proposed in this PR?
Move dependency `transformer` from `requirements.txt` to
`operator-requirements.txt`.

### Any related issues, documentation, discussions?
The dependency were introduced #2600 for supporting hugging face
operators. It should not have been a dependency for pyamber, but the
specific operator.
- #2600

This blocks #4088 

### How was this PR tested?
Existing tests.

### Was this PR authored or co-authored using generative AI tooling?
No

Author: Yicong-Huang

amber/operator-requirements.txt

@@ -22,3 +22,4 @@ pillow==10.2.0
 pybase64==1.3.2
 torch==2.8.0
 scikit-learn==1.5.0
+transformers==4.53.0

amber/requirements.txt

@@ -43,7 +43,6 @@ python-lsp-server[websockets]==1.12.0
 bidict==0.22.0
 cached_property==1.5.2
 psutil==5.9.0
-transformers==4.53.0
 tzlocal==2.1
 pyiceberg==0.8.1
 readerwriterlock==1.0.9