Reshape Targets and Mean for RHS in Cholesky solver

[turquoisedragon2926]

This is a potential fix to #2577 bug regarding Fantasy Models for Multitask GPs

Code changes

Added reshaping to align targets and fant_mean in the calculation of small_system_rhs to ensure proper tensor operations in the fantasy data update step for the GP model.

Tests

None added

Documentation

None updated

Examples

In order to test, one can run:

import torch
import gpytorch

class MultitaskGPModel(gpytorch.models.ExactGP):
    def __init__(self, train_x, train_y, likelihood, n_tasks):
        super(MultitaskGPModel, self).__init__(train_x, train_y, likelihood)
        self.mean_module = gpytorch.means.MultitaskMean(
            gpytorch.means.ConstantMean(), num_tasks=n_tasks
        )
        self.covar_module = gpytorch.kernels.MultitaskKernel(
            gpytorch.kernels.RBFKernel(), num_tasks=n_tasks, rank=1
        )

    def forward(self, x):
        mean_x = self.mean_module(x)
        covar_x = self.covar_module(x)
        return gpytorch.distributions.MultitaskMultivariateNormal(mean_x, covar_x)


input_dim = 1
output_dim = 2
n_train = 10
train_x = torch.randn(n_train, input_dim)
train_y = torch.randn(n_train, output_dim)

likelihood = gpytorch.likelihoods.MultitaskGaussianLikelihood(num_tasks=output_dim)
model = MultitaskGPModel(train_x, train_y, likelihood, output_dim)

model.train()
model.eval()

# get a posterior to fill in caches
model(torch.randn(n_train, input_dim))

# Generate some new data and get fantasy model
n_new = 5
new_x = torch.randn(n_new, input_dim)
new_y = torch.randn(n_new, output_dim)

model.get_fantasy_model(new_x, new_y)

for varying number of output_dim