Return a copy from OnebitLamb.get_lamb_coeffs (#8227)
## Problem
`OnebitLamb.step()` opens by dropping the previous step's stats in
place:
```python
# remove the previous stats
del self.lamb_coeffs[:]
```
and `get_lamb_coeffs()` handed back that same list object:
```python
def get_lamb_coeffs(self):
return self.lamb_coeffs
```
So a caller who reads the coefficients (to log them, or to watch the
trust ratios during warmup) is holding the optimizer's own list, and the
next `step()` empties it under them. The snapshot silently becomes `[]`
instead of the values that were read.
Running the two accessors as they exist today, with the one `step()`
statement that touches the list:
```
OnebitLamb (deepspeed/runtime/fp16/onebit/lamb.py)
source : return self.lamb_coeffs
before step() : [tensor(0.5000), tensor(1.5000)]
after step() : []
caller's snapshot emptied by step(): True
FusedLamb (deepspeed/ops/lamb/fused_lamb.py)
source : return lamb_coeffs
before step() : [0.5, 1.5]
after step() : [0.5, 1.5]
caller's snapshot emptied by step(): False
```
`FusedLamb` has the identical `del self.lamb_coeffs[:]` in its `step()`
and the identical accessor name, and is not affected only because its
version builds a new list on the way out. So the two optimizers disagree
today about whether the value they hand you survives the next step.
## Fix
Return a copy, so both optimizers behave the same way:
```python
return list(self.lamb_coeffs)
```
## One thing I deliberately did not change
`FusedLamb.get_lamb_coeffs` returns Python floats (`[c.item() for c in
self.lamb_coeffs]`) while this one returns the tensors. Making them
match would mean changing the element type this method has always
returned, which is a separate call from fixing the aliasing, so I left
it alone rather than folding a behaviour change into a bug fix. Happy to
align it here or in a follow-up if you would rather the two accessors
were identical.
## Test
`test_onebit_lamb_get_lamb_coeffs_returns_a_copy` in
`tests/unit/runtime/half_precision/onebit/test_onebit.py`. It needs no
accelerator and no distributed backend: the accessor is pure Python, and
since `OnebitLamb.__init__` asserts on an initialized backend, the test
builds the instance with `__new__` and gives it only the attribute the
accessor reads. It sits with the other 1-bit Lamb tests rather than in a
new file, and it runs in `cpu-torch-latest` since that job runs all of
`unit/` and this module has no accelerator-level skip.
Fails before the change (`step() emptied the list returned to the
caller`) and passes after.
## Checks
`pre-commit run --files deepspeed/runtime/fp16/onebit/lamb.py
tests/unit/runtime/half_precision/onebit/test_onebit.py` is clean,
including yapf, flake8, check-torchdist, check-license and codespell.
Commit is signed off.
Signed-off-by: Vineeth Sai <vineethsai4444@gmail.com>
