Given an array `nums` of integers, return the length of the longest arithmetic subsequence in `nums`.

Recall that a subsequence of an array `nums` is a list `nums[i1], nums[i2], ..., nums[ik]` with `0 <= i1 < i2 < ... < ik <= nums.length - 1`, and that a sequence `seq` is arithmetic if `seq[i+1] - seq[i]` are all the same value (for `0 <= i < seq.length - 1`).

Example 1:

    Input: nums = [3,6,9,12]
    Output: 4
    Explanation:
    The whole array is an arithmetic sequence with steps of length = 3.

Example 2:

    Input: nums = [9,4,7,2,10]
    Output: 3
    Explanation:
    The longest arithmetic subsequence is [4,7,10].

Example 3:

    Input: nums = [20,1,15,3,10,5,8]
    Output: 4
    Explanation:
    The longest arithmetic subsequence is [20,15,10,5].

Constraints:

- `2 <= nums.length <= 1000`
- `0 <= nums[i] <= 500`