Fix msan use-of-uninitialized-value warning in collective_ops_test.
Fix msan use-of-uninitialized-value warning in collective_ops_test.

Calls to `ScopedAllocator::AllocateRaw` respect `Allocator::kAllocatorAlignment`, i.e. they return aligned memory.  This means that some chunks of the backing tensor can be padded to achieve alignment.

However, when the op downstream of the scoped allocator consumes the padded memory buffer, it may receive uninitialized memory.  This is because no op writes to the pad region.

In this test, we were all-reducing tensors of shape [1] and type float16, which caused 48 byte padding.  Changing the tensor type to float64 fixes the msan warning because everything is now aligned to 64 bytes.

PiperOrigin-RevId: 266028337

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