How would the gravitons cause time dilation? If we consider gravity to be a particle (graviton) then how can time dilation be explained by quantum physics?  I suspect the graviton flux passing through an atom will slow down the atomic cohesion speed and velocity.  Is that a valid statement?
 A: We can have an idea of what happens, but it is necessarily an incomplete view. For a linear approximation of gravity, it is similar to the way we treat and understand the photon -- with some differences to be pointed out in a couple paragraphs below. 
The graviton is the quantum excitation of the gravitational field, which can be expressed in terms of the spacetime metric. So it is the smallest change in the metric, the quantum unit of change. A small perturbation of  the vacuum metric, classically, can be written as 
$$g_{\mu\nu} = \eta_{\mu\nu} + h_{\mu\nu}$$
With the first term on the right the Minkowski spacetime metric and the second term denoted by $h_{\mu\nu}$ denoting the perturbation. The graviton is the smallest perturbation, the quanta of the gravitational field. 
The h terms can be quantized similarly to the quantization of the electromagnetic field, with 2 spin possibilities as is true for any massless field. There are creation and annihilation operators that one can define, and thus create state vectors to define any configuration.
So, yes, a graviton will change the metric ever so slightly, and in a linear approximation we can calculate the h terms (if not too complex a situation). The metric change h can then have time dilating  (or contracting, or oscillating.  A gravitational change will usually involve a huge number of gravitons) ever so slightly. The spatial part of the metric can also change slightly. 
You get enough gravitons together and you can describe or 'create' a macroscopically observable metric change. If the metric change is physical, not simply that we changed coordinate systems, the invariant curvature will change.
Some caveats:
1- just like photons used to describe electromagnetic fields it is hard to describe complex macroscopic fields that way. But gravitational waves can be described similarly to electromagnetic waves as a superposition of many gravitons at various different frequencies. 
2- gravitational waves, and specifically gravitons, have spin 2 (photons are spin 1), and are indeed massless. They have therefore two polarizations but unlike (say the electric field of) the photon they are oscillations in two dimensions -- a gravitational wave going in the z direction will vibrate a bit like a balloon squeezed in the x axis and expanding in the y axis, and then the other way around and oscillate back and forth, or at 45 degrees, denoted respectively as  + and x polarizations. 
3- the above is a linear approximation, in a weak field. But gravity, as general relativity has it, is a nonlinear theory, so the gravitons also interact with each other (unlike, for the most part electromagnetism where photons pass by each other), and since they carry energy (equivalent to mass) they also interact with anything that has energy (or mass). To solve the nonlinear problem even in classical general relativity is not usually possible if you have many particles, or gravitons, and other bodies interacting. The nonlinear quantum theory for Gravity has not been able to be developed and be consistent -- we still don't know what strong quantum gravity really is. The linear approximation we believe is like the view expressed above. We can solve the full nonlinear classical relativity equations for certain symmetric spacetimes, like spherical or axially symmetric and stationary, and some others. The 3 body problem in general relativity is not solvable exactly.
4- to be accurate, one has to be careful in general relativity that what one calculates is not simply an artifact of the coordinate system, or of changing the gauge, so that one is calculating invariant changes in the curvature. 
5- the gravitational interaction is many orders of magnitude weaker than electromagnetism, and the effect of a graviton on an atom is too small to be measured. You'd need a hugely high freq graviton
and you'd be in the realm of very nonlinear quantum gravity before you could get anything measurable. However, it is definitely the case that a macroscopic gravitational field, many many gravitons, will have time dilation effects on atoms. The atomic transition in the cesium atom used to calculate time in the GPS satellites is indeed affected by the gravity change between there and the surface of the earth, and the times then have to adjusted to correspond to earth-surface times. It's been measured, and adjusted. 
