Given an integer array `nums`, return the number of all the arithmetic subsequences of `nums`.

A sequence of numbers is called arithmetic if it consists of at least three elements and if the difference between any two consecutive elements is the same.

- For example, `[1, 3, 5, 7, 9]`, `[7, 7, 7, 7]`, and `[3, -1, -5, -9]` are arithmetic sequences.
- For example, `[1, 1, 2, 5, 7]` is not an arithmetic sequence.

A subsequence of an array is a sequence that can be formed by removing some elements (possibly none) of the array.

- For example, `[2,5,10]` is a subsequence of `[1,2,1,2,4,1,5,10]`.

The test cases are generated so that the answer fits in 32-bit integer.

Example 1:

    Input: nums = [2,4,6,8,10]
    Output: 7
    Explanation: All arithmetic subsequence slices are:
    [2,4,6]
    [4,6,8]
    [6,8,10]
    [2,4,6,8]
    [4,6,8,10]
    [2,4,6,8,10]
    [2,6,10]

Example 2:

    Input: nums = [7,7,7,7,7]
    Output: 16
    Explanation: Any subsequence of this array is arithmetic.

Constraints:

- `1 <= nums.length <= 1000`
- `-231 <= nums[i] <= 231 - 1`