Allow for observation noise without mask in ModelListGP

[swierh]

Motivation

Fixes #2734

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

I've added a test to the unit test, in addition, a separate example:

import torch
from botorch.fit import fit_gpytorch_mll
from botorch.models.gp_regression import SingleTaskGP
from botorch.models.model_list_gp_regression import ModelListGP
from botorch.sampling.list_sampler import ListSampler
from botorch.sampling.normal import SobolQMCNormalSampler
from gpytorch.mlls.sum_marginal_log_likelihood import SumMarginalLogLikelihood

model = ModelListGP(
    *[SingleTaskGP(train_X=torch.tensor([[0.0], [1.0]]), train_Y=torch.tensor([[0.0], [1.0]])) for _ in range(2)]
)
mll = SumMarginalLogLikelihood(model.likelihood, model)
mll = fit_gpytorch_mll(mll)
sampler = ListSampler(*[SobolQMCNormalSampler(sample_shape=torch.Size([1])) for _ in range(2)])

# Works:
model.fantasize(
    torch.tensor([[0.5]]),
    sampler=sampler,
    observation_noise=torch.tensor([[0.2, 0.2]]),
    evaluation_mask=torch.tensor([[1.0, 1.0]], dtype=torch.bool),
)

# Now also works:
model.fantasize(
    torch.tensor([[0.5]]),
    sampler=sampler,
    observation_noise=torch.tensor([[0.2, 0.2]]),
)

[saitcakmak]

Thanks for the quick fix!

[facebook-github-bot]

@saitcakmak has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.99%. Comparing base (a43bd4d) to head (95a90f1).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2735   +/-   ##
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  Coverage   99.99%   99.99%           
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  Files         203      203           
  Lines       18673    18675    +2     
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+ Hits        18672    18674    +2     
  Misses          1        1           

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